2. If $a$ is a rational number, then the value of $|a|-a$ must be ( ).
(A) greater than 0.
(B) less than 0.
(C) not greater than 0.
(D) not less than 0.
Answer: D.
Solution: When $a \geq 0$, $|a|-a=a-a=0$;

When $a<0$, $|a|-a=-a-a=-2a>0$.

Therefore, the value of $|a|-a$ is definitely not less than 0.

Hence, the correct choice is D.