Is question 'b' correct

Hi teachers,
Context:
Mary: Peter! What are you thinking? We don’t have any drinks yet. Are you going to get us some drinks or are we going to be here the whole day without them?
Peter: All right! I'll get them.
Peter starts for the counter.
Mary: Peter! come back. You don't know what we want to drink, do you?

This one is OK.
a) What did Peter do after hearing Mary's complaint?
He started for the counter to get the drinks.
How about this one?
b) Why did Mary call Peter when he started for the counter?
Because he didn't know what they wanted to drink.

Thanks in advance.