$7 \cdot 112$ If $f(x)=\frac{x(x-1)}{2}$, then $f(x+2)$ equals
(A) $f(x)+f(2)$.
(B) $(x+2) f(x)$.
(C) $x(x+2) f(x)$.
(D) $\frac{x f(x)}{x+2}$.
(E) $\frac{(x+2) f(x+1)}{x}$.
(4th American High School Mathematics Examination, 1953)