Ans: B
I must be true. By the condition $ab>0$, $a$ and $b$ must be both positive or both negative. Therefore, $\frac{a}{b}>0$.
I must be true. By the condition $ab>0$, $a$ and $b$ must be both positive or both negative. Therefore, $\frac{a}{b}>0$.
II may be false. Suppose $a=-4$ and $b=-7$. It is clear that $ab=(-4)(-7) =28>0$. But $a+b = (-4)+(-7)=-11<0$.
III must be true. By the condition $ab>0$, $a$ and $b$ must be non-zero. The square of any non-zero real numbers must be positive. And the sum of two positive number must be positive.
