__________ catching the sight of policemen, she can't help shaking with fear because of that te

__________ catching the sight of policemen, she can't help shaking with fear because of that terrible experience.

A.On

B.By

C.At

D.For

答案及解析： A

解析：

On catching the sight…“一看到……”，on表时间，意为“一……（就……）”。