A large valve，usually installed above the ram preventers，that forms a seal in the annular space between the pipe and wellbore or，if no pipe is present，on the wellbore itself is called the（）.

A . open and individual tank valves open B . open and the individual tank valves closed C . closed and the individual tank valves closed D . closed and the individual tank valves ope

A . a pressure reducing valve B . a pilot valve C . an operating piston D . downstream pressure

A . inefficiency B . oiliness C . detergency D . purity

A . open and individual tank valves open B . open and the individual tank valves closed C . closed and the individual tank valves closed D . closed and the individual tank valves ope

A . open B . close C . remain in its position D . flood the evaporator with refrigerant

A . after the discharge valve B . between the pump and discharge valve C . after the suction valve D . between the pump and suction valve

A . expends B . spreads C . extends D . enlarge

A . be jammed B . be opened C . be seated D . be broke

A . closing later B . opening sooner C . staying open for a shorter period of time D . staying open for a longer period of time

A . nuts and bolts B . new joints C . spring force D . sealing ring

A . the proper back pressure B . the cooling water temperature C . correct timing and proper seating D . accurate metering and the exhaust temperature

A . evaporator pressure decrease B . evaporator coil over-feed C . evaporator capacity increase D . expansion valve diaphragm rupture

A . direct action of cam shaft B . compressed air pressure C . hydraulic “push rods” D . direct action of the main piston moving dow

A . at B . on C . i

A . dial indicator B . outside caliper C . bridge gauge D . tram rod

A . increase back pressure during the exhaust process B . facilitate periodic replacement of the valves C . reduce the pumping loss associated with scavenging D . reduce tension on valve spring

A . return to a neutral position B . flood the evaporator C . open D . close

A . main stem stop valve B . auxiliary steam stop valve C . feed check or control valve D . blow down valve

Using placebos to test drugs sometimes has a surprising result. Researchers say people taking the placebo often report improvements in their health. This is known as "the placebo effect"--pain that is eased or stopped by an inactive substance. In such testing, the drug must perform. better than the placebo to prove that it is effective. Doctors have reported that the placebo effect can be used in treatment. For example, a doctor tells a patient that a new drug will stop the pain in his leg. The pill is only sugar. But the patient does not know that. He takes the pill and says his pain is gone. Scientists are beginning to discover some physical reasons for this reaction in some people. They are learning that much of what people believe to be true comes from what the brain expects is going to happen. If the brain believes a drug will ease pain, the brain may begin physical changes in the body that can cause the expected effect. A recent examination of studies on drugs for depression found that placebos eased the depression about as well as the active drugs. Other studies have explored the power of placebos. A study in Japan involved thirteen people who reacted to the poison ivy (常青藤)plant. Poison-ivy causes red itchy sores(伤痕)on some people who touch it. Each person was rubbed on one arm with a harmless leaf, but was told it was poison ivy. Each person was then touched on the other arm with poison ivy, but was told it was a harmless leaf. All thirteen people developed a reaction on the arm where the harmless leaf touched their skin. Only two reacted to the poison ivy leaves. Doctors and scientists worry that the use of placebos may not always be harmless. They say people can become victims of false doctors and others who use placebos to claim they can cure disease. What do medical researchers usually use to make placebo pills？ A．Ivy leaves. B．Harmless leaves. C．Medicine being tested. D．sugar.

One place where children (3)_____ A-characteristics is school, which is, by its very nature, a highly competitive institution. Too many schools adopt the 'win at all costs' moral standard and (4)_____ their success by sporting (5)_____ The current passion for making children compete against their classmates or against the clock produces a two-layer system, in which competitive A-type seem in some way better than their B-type fellows. Being too (6)_____ to win can have dangerous (7)_____: remember that Pheidippides, the first marathon runner, dropped (8)_____ seconds after saying: "Rejoice, we (9)_____!" (10)_____ the worst form. of competition in schools is the disproportionate emphasis on examinations. It is rare that school (11)_____ pupils to concentrate on those things they do well. The merits of competition by examination are somewhat (12)_____, but competition in the certain knowledge of failure is positively harmful. Obviously, it is neither practical nor (13)_____ that all A-youngsters change into B's. The world needs different kinds of person, and schools have an important duty to try to fit a child's personality to his possible future (14)_____ It is top management. If the (15)_____ of schools with academic work was (16)_____, more time might be spent teaching children surer values. Perhaps selection for the (17)_____ professions, especially medicine, could be made less by good grades in chemistry and more by such (18)_____ as (19)_____ and sympathy. It is surely a mistake to choose our doctors exclusively from A-type stock. B's are important and should be (20)_____. A．prevailing B．profound C．precautionary D．progressive