Why can the hummingbird hover and even fly backwards？
A. Because its wings are large enough.
Because it can tilt its body.
C. Because it has whirling and horizontal blades like a helicopter.
D. Because it can produce down-draft of air by beating their wings very fast.
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Which of the following is the reason for the Crash in 1929？
A. High taxation for trade.
B. Tricks of some businessman.
C. Speculation of some people.
D. Strict regulation of the state.
The first known census report took place in Babylonia in_____.
A. 1085
B. 1200 B.C.
C. 3800 B.C.
D. 1790
The American census today costs the government approximately_____.
A. $ 0. 0/person
B. $ 0. 10/person
C. $1. 00/person
D. $ 10. 00/person
计算机操作系统中，若WAlT、SIGNAL操作的信号量S初值为3，当前值为-2，则表示当前有()个等待信号量S的进程。
A. 1
B. 2
C. 3
D. 0
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