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【单选题】
第一心音的产生是由于A.
动脉瓣关闭
B.
动脉瓣开放
C.
房室瓣关闭
D.
房室瓣开放
E.
房室瓣开放和动脉瓣关闭血液撞击动脉壁
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举一反三
【单选题】We drop a piece of metal into a jar full of water, the metal will float if it weighs less than______.
A.
an equal amount of silver
B.
an equal amount of gold
C.
all the water in the jar now
D.
the water that leaves the jar
【单选题】Transportation in the U.S. Railroads are paths of parallel metal rails that allow a wheeled vehicle to move more easily by reducing friction. Iron mils were first used in the early 1800s to guide hors...
A.
Y
B.
N
C.
NG
【简答题】A. cycled B. drove C. wheeled D. fled
【单选题】This kind of metal ______ well，and therefore the metal stick ______ into halves easily. [ ]
A.
is cut，is cut
B.
cuts，cuts
C.
is cut，cuts
D.
cuts，is cut
【简答题】间歇操作的设备要排列工艺 和 。
【单选题】Metal-wheeled chariots are the_________of the tanks of modern warfare.
A.
prototype
B.
original
C.
model
D.
sample
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