折射定律，入射角的正弦跟折射角的正弦成（）。
A.正比
B.反比
C.分式
D.等式
参考答案：A
从1,2,3,4,5这5个数中任取两个,则这两个数正好相差1的概率是________。
Only when he returned _________ that the war had ended and the army had come back.
A．we tell
B．we told
C．did we tell
D．were we told
