3. If the length of a rectangle is increased by $p \%$, to keep the area of the rectangle unchanged, the width of the rectangle should be reduced by ( ).
(A) $p \%$
(B) $(1-p) \%$
(C) $\frac{p}{1+p} \%$
(D) $\frac{100 p}{100+p} \%$
3. D Hint: Let the length of the rectangle be $a$, and the width be $b$. Then according to the selection, the length increases to $a(1+p) \%$. And the width decreases by $x \%$. That is, the width is $b(1-$ $x \%)$, and the area remains unchanged, so $a(1+p \%) b(1-x \%)=a b$.