Not one of the potential investors is expected to make an offer to buy Frist Interstate Bank until a merger agreement is signed that includes a provision for penalties if the deal were not to be concluded

I chose:
is expected to make an offer to buy Frist Interstate Bank until a merger agreement is signed that includes a provision for penalties if the deal was

But the correct answer is A, as in the prompt with were

Why is were correct? Isnt deal singular?