Not sure if this is the right area for combinations/pascal triangle questions...

A)What is a constant in Binomial expansion? Example, I have to state whether or not a constant appears in the expansion of (x+y)^11, and explain why. A quick explanation, pls?

B) Sandra is testing to see if she can tell the difference between diet pop, and regular pop. She knows there are 3 cups of diet pop, and 2 cups of regular pop, and she'll write the order which she believes the pop to be in. How many ways can record D(for Diet) and R(for Regular) if order matters?

C)There are 8 non-fiction books, and 7 fiction books. You are to recieve two of them as a gift, how many combinations are possible if you are to recieve AT LEAST 1 fiction book?

-Is this as simple just figuring out how many combinations of 2 fiction books there are, and multiplying it by the combinations of 1 fiction book, 1 non-fiction book(which would be 56 right?)?

D) How do you solve for a constant tern in the expansion of (x+2)^9?

Mucho Gracias to any and all help!