$A$ deuteron contains

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$ : Hydrogen is the most abundant element in the Universe,but it is not the most abundant gas in the troposphere.
Reason $R$ : Hydrogen is the lightest element.
In the light of the above statements,choose the correct answer from the options given below:

In which of the following reactions,hydrogen is evolved?
$I$. Reaction of sodium borohydride with iodine
$II$. Oxidation of diborane
$III$. Reaction of boron trifluoride with sodium hydride
$IV$. Hydrolysis of diborane

Deuterium resembles hydrogen in chemical properties but reacts:

Give the reaction of methane with steam for the production of dihydrogen.

The oxidation number of hydrogen in $MH_2$ is