Which of the following species can act as both a $Br\text{ø}nsted$ acid and a $Br\text{ø}nsted$ base?

For the reaction $NH_4^+ + S^{2-} \rightleftharpoons NH_3 + HS^-$,identify the nature of $NH_3$ and $HS^-$ in the context of the Brønsted-Lowry theory.

Provide one example each of an acid,a base,and a salt that are available in nature.

The strongest Bronsted base among the following anions is:

The values of dissociation constants $(K_a)$ of some acids (at $25\,^{\circ}C$) are as follows. Indicate which is the strongest acid in water:
$(A)$ $1.4 \times 10^{-2}$
$(B)$ $1.6 \times 10^{-4}$
$(C)$ $4.4 \times 10^{-10}$
$(D)$ $4.3 \times 10^{-7}$

When a solution of $NaOH$ is left exposed to air for a few hours,what happens?