Given below are two statements one is labelled as Assertion A and the other is labelled as Reason R:
Assertion A : The amphoteric nature of water is explained by using Lewis acid/base concept.
Reason R : Water acts as an acid with $\mathrm{NH}_3$ and as a base with $\mathrm{H}_2 \mathrm{~S}$.
In the light of the above statements choose the correct answer from the options given below