giúp mik vs mik đag cần gấp.

$\displaystyle \begin{array}{{>{\displaystyle}l}} Với\ a >0,b >0\\ \sqrt{\frac{a}{b}} +\frac{a}{b}\sqrt{\frac{b}{a}} =\sqrt{\frac{a}{b}} +\sqrt{\frac{a}{b}} .\sqrt{\frac{a}{b}} .\sqrt{\frac{b}{a}} =\sqrt{\frac{a}{b}} +\sqrt{\frac{a}{b}} .\sqrt{\frac{a.b}{b.a}}\\ =\sqrt{\frac{a}{b}} +\sqrt{\frac{a}{b}} .1=2\sqrt{\frac{a}{b}} =2\frac{\sqrt{a} .\sqrt{b}}{b} =\frac{2\sqrt{ab}}{b}\\ \Rightarrow Chọn\ D \end{array}$