집합의 뜻과 포함 관계

Assume the universal set is the set of real numbers. Let

$$\begin{aligned}A & = \left\{ { - 3, - 2,2,3} \right\}\!, & B & = \left\{ {\left. {x \in \mathbb{R}} \right|x^2 = 4\text{ or } x^2 = 9} \right\}\!, \\ C & = \left\{ {x \in \mathbb{R}\left. \right|x^2 + 2 = 0} \right\}\!, & D & = \left\{ {x \in \mathbb{R}\left. \right|x > 0} \right\}\!\end{aligned}$$

. Respond to each of the following questions. In each case, explain your answer.

Is the set $C$ a subset of the set $D$?