Which of the following is an oxyacid?

Observe the following species: $AlCl_3, NH_3, H^{+}, Co^{3+}, OH^{-}, H^{-}, Mg^{2+}, BF_3, Cl^{-}$. How many Lewis bases are present in the above list?

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 $NH_{3}$ and as a base with $H_{2}S$.
In the light of the above statements,choose the correct answer from the options given below:

$HSO_4^-$ is the conjugate base of

The strongest Bronsted base among the following anions is:

In the equilibrium $HClO_4 + H_2O \rightleftharpoons H_3O^{+} + ClO_4^-$,which of the following statements is correct?