China's population is expected to increase to about 1.5 billion in 2040 it will start to

问题补充：

Chinas population is expected to increase to about 1.5 billion in 2040, it will start to decline slowly.

A. where

B. which

C. on which

D. after which

答案：

A. where

B. which

C. on which

(答案→)D. after which

解析：考查非限制性定语从句。句中有逗号，说明是非限制性定语从句，先行词是2040，指时间，排除A；on when相当于when，但on指具体的一天，这里的2040年是一段时间，排除C；which作主语或宾语，it will start to decline slowly不缺少成分，排除B，after which相当于after 2040，故选D。