3.4 The non-zero real numbers $(a, b)$ that satisfy the equation $\frac{1}{a}+\frac{1}{b}=\frac{1}{a+b}$, such $(a, b)$
(A) None.
(B) 1.
(C) 2.
(D) There is one pair for each $b \neq 0$.
(E) There are two pairs for each $b \neq 0$.
(31st American High School Mathematics Examination, 1980)