Charlie's Utility Function Is U(A, B)= AB, Where a and B

Charlie's utility function is U(A, B) = AB, where A and B are the numbers of apples and bananas, respectively, that he consumes.If Charlie is consuming 15 apples and 30 bananas, then if we put apples on the horizontal axis and bananas on the vertical axis, the slope of his indifference curve at his current consumption is

A) -16.
B) -2.
C) -4.
D) -1/2.
E) -1/4.