The $\mathrm{ortho}\mathrm{ }$and $\mathrm{para}\mathrm{ }$hydrogen differ in respect of which of the following?

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:

Which of the following statements about hydrogen is incorrect?

Write a chemical equation for the reaction taking place in the flask. Write name and one property of the gas evolved.

Out of the oxides given below which of them cannot be reduced by ${\mathrm{H}}_2$?

$\mathrm{I}-{\mathrm{Al}}_2{\mathrm{O}}_3; \mathrm{II}-\mathrm{CuO}; \mathrm{III}-\mathrm{ZnO}$