China a lot of people moving to urban areas in the last 20 years. However many migrants

问题补充：

China a lot of people moving to urban areas in the last 20 years. However, many migrants left their families behind.

A. saw

B. sees

C. has seen

D. had seen

答案：

A. saw

B. sees

(答案→)C. has seen

D. had seen

解析：考查时态：句意：在过去的中中国见证了很多人搬到都市，然而，很多移民把家人留下了。时间状语是in the last 20 years表示从过去到现在一直在做的事情，用现在完成时，选C。