Convert the following rectangular coordinates into polar coordinates. Always choose $0 \le \theta < 2\pi$.

(a) $(0,5)$
$r =$ , $\theta =$ .

(b) $(-1,-\sqrt 3)$
$r =$ , $\theta =$ .

(c) $(-\sqrt 3, -1)$
$r =$ , $\theta =$ .

(d) $(4,-3)$
$r =$ , $\theta =$ .