Concerning (a), the first times you encounter it, an _a fortiori argument_ can be tricky, despite its simplicity. You know $a b = ba$ holds _for all elements $a, b \in G$_ , and the elements of $H$ are just _some of the elements of $G$_.
Concerning (a), the first times you encounter it, an _a fortiori argument_ can be tricky, despite its simplicity. You know $a b = ba$ holds _for all elements $a, b \in G$_ , and the elements of $H$ are just _some of the elements of $G$_.