1 .
Consider the equation
$$
x_1 + x_2 + x_3 + x_4 = 33,
$$
where $x_1 \geq 0$, $x_2 \geq 0$, $x_3 \geq 0$, $x_4 \geq 0$ are integers. How many solutions does this
equation have?
(a)
${36 \choose 3}$
(b)
${37 \choose 4}$
(c)
${37 \choose 3}$
(d)
${36 \choose 4}$