$(10$pt$)$ False or true?

$q,$ Let A and $B$ be two sets. It is always the case that:

$|AB|=|A|+|B|$

$q,$ Let A and $B$ be two sets. It is always the case that:

$AB=BA$

$q,$ Let $f$:$xY$ be an invertible function. It is always the case that $f$@$f^{-1}$ is the identity function on $x.$

$q,$ An algorithm whose running time is $(n^2)$ is inefficient compared with another algorithm whose running time is $O(nlogn).$

$q,$ Let $R$ be a relation. If $R$ is not irreflexive, then it is reflexive.

$1$