Asel says: I don't remember where I've left my key of my flat. I think I ____it on my...

Question

Asel says: I don't remember where I've left my key of my flat.
I think I ____it on my desk at school,I'm sure.
a)must have left
b)might have left
c)should have left

Если можете пожалуйста объясните, почему

Answer 1

С там в прошедшем времени вроде