Question

The pen ___ he wrote was broken

Answer 1

The pen _with which__ he wrote was broken

定语从句可改写为：he wrote with the pen,先行词是物,所以用with which

The age __at which___ children go to school is seven

定语从句可改写为：children go to school at the age,先行词是物,所以用at which

She has three children,all__of whom___ are at school

定语从句可改写为：all of the children are at school,先行词是人,所以用of whom