For Every Gift That a Gives to B from the First

For every gift that A gives to B from the first to the tenth gift,A receives a net benefit of $10.The additional cost to A of giving an additional gift is constant at $5.It follows that A's marginal benefit curve for giving gifts to B is ___________________ (assuming that we place "marginal benefits" on the vertical axis and "number of gifts" on the horizontal axis) .

A) downward-sloping over units 1 -10.
B) upward-sloping over units 1-10.
C) horizontal over units 1-10.
D) vertical over units 1-10.
E) There is not enough information provided to answer the question.