Given $x,y,a,b$ are negative real numbers such that $x<a<0$ and $y<b<0$. How many of the following inequalities must be true?
(I) $x+y<a+b$
(II) $x^2>a^2$
(III) $\frac{1}{x}>\frac{1}{a}$
(IV) $\frac{x}{y}<\frac{a}{b}$