题干
—Has Mr. Black decided not to take part in the party?
—Yes. But the reason______he refused the invitation is not clear.
A：what
B：for which
C：with what
D：which
答案(点此获取答案解析）
B
s时的波动图象。已知波速为v=20m/s．在t=0时刻，x=10m的质点的位置和速度方向可能是 