'but for' structure?

I ran into difficulties answering the following test question: (I guess it's B but not sure)

1. Which clause could not be joined with the second, given clause?

___ , the hospital could have burned down.

A) If the nurse hadn't had a presence of mind

B) But for the nurse's presence of mind

C) Had the nurse not had a presence of mind

D) Hadn't the nurse had a presence of mind

2. Which sentence is incorrect? The boss demanded that the employees__ .

a) do overtime b) did do overtime c) should do overtime d) should have done overtime

(my guess it's b)

3. (I guess it's C)

Which sentence is incorrect__ , I would have got sunburnt.

A) If I hadn't had suntan lotion with me

B) Hadn't I had suntan lotion with me

C) Had it not been for the suntan lotion

D) Had I not had suntan lotion with me

Thank you very much.

Palinkasocsi