[Grammar] Must be or must have been?

Who did it?

It must be Nancy. / It must have been Nancy.


Please tell me which a more proper, or both are alright?

Thanks.

[bhaisahab]

Originally Posted by Unregistered

Who did it?

It must be Nancy. / It must have been Nancy.


Please tell me which a more proper, or both are alright?

Thanks.

'Who did it?' refers to a past action so 'It must have been Nancy' is the correct choice.