4.5 If $a \geqslant 2, \quad b>2$, then which of the following expressions is correct?
(A) $a b < a+b$.
(B) $a b \leqslant a+b$, equality can be achieved.
(C) $a b > a+b$.
(D) $a b \geqslant a+b$, equality can be achieved.
(1st "Five Sheep Cup" Junior High School Mathematics Competition, 1989)
[Solution] From $\frac{a+b}{a b}=\frac{1}{a}+\frac{1}{b}0, a b>0$, then $a+b<a b$. Therefore, the answer is $(C)$.