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设凸k边形的对角线的条数为f(k),则凸k+1边形的对角线的条数f(k+1)等于( )
A.f(k)+k-1
B.f(k)+k
C.f(k)+k+1
D.f(k)+k-2
假设凸k边形的对角线有f(k)条,则凸k+1边形的对角线条数为f(k)+_________.