Cardinality of $A = \varnothing, B = \{ \varnothing \}, C = \{\{\varnothing\}\}$ Given three sets $A = \varnothing, B = \{ \varnothing \}, C = \{\{\varnothing\}\}$ what are the cardinalities of those sets ?
Obviously cardinality of $A$ is $0$ and cardinality of $B$ is $1$, but I am not sure about set $C$, because some sources say that cardinality of such set is $2$. Can you please clarify this to me ?

The cardinality of $A$ is 0. The cardinality of $B$ and $C$ is both 1. For $B$ it is clear. $C$ just contains one element as well. This is the set that contains the emptyset $\emptyset$. Namely $B$. One could write $C=\{B\}$, which makes it clear.
