When you can't reach him at this number,_________ his home number.

问题描述:

When you can't reach him at this number,_________ his home number.
A try dialing B try to dial C trying dialing D trying to dial
选哪个,为什么,

A
首先排除C/D,因为不是句子,而when you can't reach him at this number为从句,故需一个主句.
try doing ...试着做.
try to do .努力做.