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
网友回答
A. saw
B. sees
(答案→)C. has seen
D. had seen
解析：考查时态：句意：在过去的20年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。