A．sinceB．whileC．whetherD．when
A. since
B. while
C. whether
D. when
A．They smile to cover embarrassment.B．It is an unusual and even suspicious behavior.C．
A. They smile to cover embarrassment.
B. It is an unusual and even suspicious behavior.
C. They smile to show politeness.
D. It is an expression of pleasure.
【S13】
A. over
B. out
C. off
D. away
A．importantB．commonC．excitingD．unusual
A. important
B. common
C. exciting
D. unusual
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