It is reported that two schools，________ are being built in my hometown，will open next year.
A．they both B．which both
C．both of them D．both of which

选D.both of which

本句的主干是：
It is reported that two schools will open next year.
(其中that引导的是一个主语从句，并且这个主语从句又是一个带有定语从句的复合句）。
而剩下的中间部分则是定语从句，修饰先行词schools。
（整个主语从句是： two schools，both of which are being built in my hometown，will open next year. )


这样我们能从both of 结构可以排除A和B答案，但C也不行，是因为从句是需要有连词引导的，若选C，则全句无连词来连接该定语从句。

或者换个角度说，如果选择了C答案，那么我们发现，在这个主语从句里将有两处谓语了（are being built和will open）。在同一个句子中是不能同时有两个谓语的，除非其中一个是从句的谓语，所以，必须选D答案才能使本句正确。