The name hydrogen was given by

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A:$ Hydrogen is an environment-friendly fuel.
Reason $R:$ Atomic number of hydrogen is $1$ and it is a very light element.
In the light of the above statements,choose the correct answer from the options given below:

$A$ deuteron contains

Hydrogen has three isotopes. The number of possible diatomic molecules will be:

Which of the following combinations represents water gas?

Which of the following has the highest calorific value?