The strongest base is

Given below are two statements:
Statement $I$: The halogen that makes the longest bond with hydrogen in $HX$ has the smallest covalent radius in its group.
Statement $II$: $A$ group $15$ element's hydride $EH_3$ has the lowest boiling point among corresponding hydrides of other group $15$ elements. The maximum covalency of that element $E$ is $4$.
In the light of the above statements,choose the correct answer from the options given below.

Which of the following compounds is a tribasic acid?

In the catalytic oxidation of ammonia,an oxide is formed which is used in the preparation of $HNO_3$. This oxide is

$A + H_2O \to B + HCl$ and $B + 3H_2O \to C + 3HCl$. Compounds $(A)$,$(B)$,and $(C)$ are respectively:

Which gas,when absorbed by ferrous sulfate,produces a dark brown color?