问题
选择题
— Can you teach me ping-pong? — Sure.[ ]
A. play
B. to play
C. plays
D. playing
答案
答案：B
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— Can you teach me ping-pong? — Sure.[ ]
A. play
B. to play
C. plays
D. playing
答案：B
时的波形曲线如图2-13所示。若振动以余弦函数表示，且本题各点振动初相取-π到π之间的值，则()。
