Number of nucleons in $D_2$ molecule is

Given below are two statements,one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A:$ Isotopes of hydrogen have almost same chemical properties,but difference in their rates of reaction.
Reason $R:$ Isotopes of hydrogen have different enthalpy of bond dissociation.
In the light of the above statements,choose the most appropriate answer from the options given below:

Give the chemical reactions for the preparation of water gas and producer gas.

Which of the following oxides cannot be reduced by hydrogen?

$BeH_2$ can be prepared by the reaction of

What is the structural formula of lithium tetrahydridoaluminate?