$13 \cdot 17$ When $a, b$ are two unequal positive numbers, among the following three algebraic expressions:
甲: $\left(a+\frac{1}{a}\right)\left(b+\frac{1}{b}\right)$, 乙: $\left(\sqrt{a b}+\frac{1}{\sqrt{a b}}\right)^{2}$,
丙: $\left(\frac{a+b}{2}+\frac{2}{a+b}\right)^{2}$
the one with the largest value
(A) must be 甲.
(B) must be 乙.
(C) must be 丙.
(D) is generally not certain, and depends on the values of $a, b$. (China High School Mathematics League, 1982)