What is the cardinality of each of the following sets?

(a) $\emptyset$

(b) $\lbrace \emptyset \rbrace$

(c) $\lbrace \emptyset, \lbrace \emptyset \rbrace \rbrace$

(d) $\lbrace \emptyset, \lbrace \emptyset \rbrace, \lbrace \emptyset, \lbrace \emptyset \rbrace \rbrace \rbrace$