China’s population is expected to increase to about 1.5 billion it will start to decline

问题补充：

China’s population is expected to increase to about 1.5 billion, it will start to decline slowly.

A. after that

B. since when

C. on which

D. after which

答案：

A. after that

B. since when

C. on which

(答案→)D. after which

解析：句意：中国人口预计将增加到大约15亿,之后它将开始缓慢下降。句子中间逗号隔开，逗号钱前是一个主句，逗号后没有连词应是一个从句。that 引导定语从句时不与介词连用，after 是介词，故A.排除。since 引导的状语从句，主句应用现在完成时，故B项排除。根据句意15亿以后，人口开始下降。故选D。