Select all the true statements
Select one or more:
a. $\exists a \in \{x \in \mathbb{R} \mid x \ge 2 \text{ and } x \le 3\} \text{ such that } a \ge 0$.
b. $\{1, 1, 2, 3, 4\} \subseteq \{4, 3, 1, 1, 2\}$
c. $\{\emptyset\} = \emptyset$
d. The collection of all subsets of $\{a, b\}$ is $\{\{a\}, \{b\}, \{a, b\}\}$
e. The set of all real numbers is a subset of the set of complex numbers.