Why does hydrogen occur in a diatomic form rather than in a monoatomic form under normal conditions?

Which of the following statements are $NOT$ correct?
$A$. Hydrogen is used to reduce heavy metal oxides to metals.
$B$. Heavy water is used to study reaction mechanism.
$C$. Hydrogen is used to make saturated fats from oils.
$D$. The $H-H$ bond dissociation enthalpy is lowest as compared to a single bond between two atoms of any elements.
$E$. Hydrogen reduces oxides of metals that are more active than iron.
Choose the most appropriate answer from the options given below:

Choose the incorrect statement for deuterium.

Hydrogen readily combines with metals and thus shows its

Carbongene has $X\%$ of $CO_2$ and is used as an antidote for poisoning of $Y.$ Then,$X$ and $Y$ are

Which of the following chemicals contains chlorine?