7.92 Let $f(x)$ be a function that satisfies the following two properties:
(a) For any two real numbers $x$ and $y$, $f(x+y)=x+f(y)$, and (b) $f(0)=2$.

Then the value of $f(1998)$ is
(A) 0.
(B) 2.
(C) 1996.
(D) 1998.
(E) 2000.
(49th American High School Mathematics Examination, 1998)
[Solution] From the given, we know that $f(x)=f(x+0)=x+f(0)=x+2$, therefore $f(1998)=1998+2=2000$.
Thus, the answer is $(E)$.