题目

He looked around and caught a man ______ his hand into the pocket of a passenger.      A. put        B. to be putting       C. to put        D. putting       答案：D弦乐器发出的声音是靠______产生的，音调的高低与弦的______、______、______、______有关．弦乐器通常有一个木制的______来使声音更洪亮．