AHSME 1968 Problem 5

If $f(n)=\tfrac{1}{3} n(n+1)(n+2)$, then $f(r)-f(r-1)$ equals: $\text{(A) } r(r+1) \text{(B) } (r+1)(r+2) \text{(C) } \tfrac{1}{3} r(r+1) \\ \text{(D) } \tfrac{1}{3} (r+1)(r+2) \text{(E )} \tfrac{1}{3} r(r+1)(2r+1)$