Convert to the polar form $r e^{i \theta} .$ For Problems 15 and 16, choose $\theta$ in degrees, $-180^{\circ} < \theta \leq 180^{\circ} ;$ for Problems 17 and 18 choose $\theta$ in radians, $-\pi < \theta \leq \pi$ Compute the modulus and arguments for parts $A$ and $B$ exactly; compute the modulus and argument for part C to two decimal places.
$(\mathrm{A})-1+i \sqrt{3}$
(B) $-3 i$
$(C)-7-4 i$