$\{a,e\}\subset \{x:$$x$ is a vowel in the English alphabet $\}$

$\{1,2,3\}\subset \{1,3,5\}$

$\left\{a\right\}\subset \{a,b,c\}$

$\left\{a\right\}\in \left\{a,b,c\right\}$

Let $A=\left\{1,2,\left\{3,4\right\},5\right\}.$ Check if the following statement is incorrect or not and justify your answer.

$\left\{3,4\right\}\in A$