A star is observed at lower transit. The line of position derived from this sight is（）.

A . stars B . Moon C . Sun D . Planet

A . Southwest，clockwise to northwest B . Northeast，clockwise to west-southwest C . Northeast，counterclockwise to northwest D . East，counterclockwise to south-southwest

A . the declination is the opposite name to the latitude B . the algebraic sum of the colatitude and declination exceeds 90 C . the observer is in high latitudes above either polar circle D . the body is circumpolar

A . perform the Heimlich maneuver B . immediately start CPR C . give back blows and something to drink D . allow the person to continue coughing and dislodge the obstruction on his ow

A . The diesel oil must never be allowed to mix with the heavy fuel B . The temperature of the fuel from the pre–heater should be gradually reduced after switching over the three–way valve C . The heating steam to the pre-heater should be secured as soon as the diesel fuel passes through the three-way valve D . The heating steam must be secured before the diesel oil passes through the three-way valve

A . the Sun B . stars C . planets D . All of the above

A . has a declination equal to twice that of the Sun B . comprises two stars that appear close together C . is twice as bright as a single star D . suddenly becomes much brighter and then fade

A . come right B . sound the danger signal C . continue on your course D . come left

A . south celestial pole B . north celestial pole C . observer's meridian D . Greenwich meridia

A . Mars B . the Moon C . Jupiter D . All of the above

A . obscured B . darkened C . greyed D . delighted

A . low viscosity index B . high viscosity index C . high neutralization number D . low demulsibility quality

A . Report any leakage of cargo B . Make sure the rake ends of the barge are completely dry and mopped C . Ascertain that the hoses to be used are in good order D . Be on the lookout for work being accomplished ashore in the vicinity of the barge

A . 2.5 times as bright as a second magnitude star B . 3 times as bright as a second magnitude star C . 5 times as bright as a second magnitude star D . 10 times as bright as a second magnitude star

A . reflects B . reflected C . is reflected D . is reflecting

A . be circumpolar B . have a declination equal to or greater than your latitude C . have a GHA of 180° D . have the SHA equal to or less than the LHA

A . in spite of B . irrespective C . in respect D . in matter of

A . northeast B . west-southwest C . northwest D . south-southeast

A . left，slightly behind him B . right，slightly behind him C . left，slightly in front of him D . right，slightly in front of him

A . in distress B . approaching a bend in the channel C . operating astern propulsion D . intending to overtake another vessel

A . local hour angle B . azimuth angle C . meridian angle D . longitude

In practice, what often happens is that a new theory is devised that is really an extension of the previous theory. For example, very accurate observations of the planet Mercury revealed a small difference between its motion and the predictions of Newton's theory of gravity. Einstein's general theory of relativity predicted a slightly different motion from Newton's theory. The fact that Einstein's predictions matched what was seen, while Newton's did not, was one of the crucial confirmations of the new theory. However, we still use Newton's theory for all practical purposes because the difference between its predictions and those of general relativity is very small in the situations that we normally deal with. (Newton's theory also. has the great advantage that it is much simpler to work with than Einstein's ! ) It turns out to be very difficult to devise a theory to describe the universe all in one go. Instead, we break the problem up into bits and invent a number of partial theories. Each of these partial theories describes and predicts a certain limited class of observations, neglecting the effects of other quantities, or representing them by simple sets of numbers. It may be that this approach is completely wrong. If everything in the universe depends on everything else in a fundamental way, it might be impossible to get close to a full solution by investigating parts of the problem in isolation. Nevertheless, it is certainly the way that we have made progress in the past. The classic example again is the Newtonian theory of gravity, which tells us that the gravitational force between two bodies depends only on one number associated with each body, its mass, but is otherwise independent of what the bodies are made of. Thus one does not need to have a theory of the structure and constitution of the sun and the planets in order to calculate their orbits: Today scientists describe the universe in terms of two basic partial theories-the general theory of relativity and quantum mechanics. They are the great intellectual achievements of the first half of this century. Unfortunately, however, these two theories are known to be inconsistent with each other-they cannot both be correct. One of the major endeavours in physics today, is the search for a new theory that will incorporate them both-a quantum theory of gravity. We do not yet have such a theory, and we may still be long way from having one, but we do already know many of the properties that it must have. According to the passage, why can't any physical theory be permanently established？ A．Such a theory is only suggested as a possible way of explaining an idea. B．The person proposing such a theory may be incompetent. C．Observations always disagree with predictions. D．Observations are always falsified by predictions.

A．It goes LOW at t=0 and back HIGH at t=3ms.该脉冲在t=0ms降为低电平，t=3ms时返回高电平 B．It goes LOW at t=0.8ms and back HIGH at t=3ms.该脉冲在t=0.8ms降为低电平，t=3ms时返回高电平 C．It goes LOW at t=0.8ms and back HIGH at t=1ms.该脉冲在t=0.8ms降为低电平，t=1ms时返回高电平 D．It goes HIGH at t=0.8ms and back LOW at t=1ms.该脉冲在t=0.8ms升为高电平，t=1ms时返回低电平