1 .
How many solutions are there to the equation
$$
x_1 + x_2 + x_3 + x_4 + x_5 = 28,
$$
where $x_1 \geq 0$, $x_2 \geq 0$, $x_3 \geq 0$, $x_4 \geq 0$, and
$x_5 \geq 0$ are integers?
(a)
${32 \choose 4}$
(b)
${32 \choose 5}$
(c)
${33 \choose 4}$
(d)
${33 \choose 5}$