My niece had to change the following to Passive Voice:

Who broke the vase?

She answered: The vase was broken by whom?

The teacher says this is ungrammatical and the answer should be: Whom was the vase broken by?

Can someone explain the difference between my niece's answer and the teacher's and exactly why is it incorrect.

Thanks in advance

My niece had to change the following to Passive Voice:

Who broke the vase?

She answered:

The vase was broken by whom?

The teacher says this is ungrammatical and the answer should be:

Whom was the vase broken by?

Can someone explain the difference between my niece's answer and the teacher's and exactly why is it incorrect.

Thanks in advance