An example of a Lewis acid is

For two acids $A$ and $B$,$pK_a$ values are $1.2$ and $2.8$ respectively. Which of the following statements is true?

In the reaction $NH_4^+ + S^{2-} \rightarrow NH_3 + HS^-$,what are $NH_3$ and $HS^-$ with respect to the $NH_4^+$ and $S^{2-}$ species?

Write the conjugate acids for the following Bronsted bases: $NH_{2}^{-}$,$NH_{3}$ and $HCOO^{-}$

Give the conjugate acid of the following species: $HS^{-}, NH_{3}, C_{6}H_{5}COO^{-}, OH^{-}$

Explain the statement: "$NH_3$ acts as both a Lewis base and a Brønsted-Lowry base,whereas $BF_3$ acts only as a Lewis acid but not as a Brønsted-Lowry acid."