右移运算符是：（ ）
A. <
B. >
C. <<
D. >>
unsigned char、unsigned int等类型的长度是（ ）
A. 固定的
B. 由用户自己定义的
C. 任意的
D. 与机器字的长度有关的
Having a trip abroad is certainly good for the old couple, but it remains _______ whether they will enjoy it.
A. to see
B. to be seen
C. seeing
D. Seen
The research is so designed that once _______ nothing can be done to change it.
A. begins
B. having begun
C. beginning
D. Begun
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