Given the nature of the question, I'm sure zaheer has managed to answer part b.
Zaheer, you really need to go back to your teacher and explain that you're having problems answering the question. MS questions like this are far from rocket science and you've failed to grasp some pretty fundamental stuff here.
Providing you with the answer on AB would not help your understanding of MS calculations which is why I'm reluctant to give you the answer. Go through it again with your teacher - I'm sure you'll understand it before very long.