单项选择题
()with the size of the whole earth, the highest mountains do not seem high at all.
A.If you compare B.Compare them C.When compared D.A comparison
单项选择题 It is useful to science()the earth as an object in space.
单项选择题 We must()that the experiment is controlled as rigidly as possible.
单项选择题 Be careful,()you'll fall down.