What is heavy water?

Hydrogen gas is prepared by using $C$ (coke) and steam in the presence of $Fe_2O_3/Cr_2O_3$. This process is known as:

Consider the reactions:
$I$. $CO + 3 H_2 \xrightarrow{A} CH_4 + H_2 O$
$II$. $CO + 2 H_2 \xrightarrow{B} CH_3 OH$
$III$. $O_2 + 2 H_2 \xrightarrow{C} 2 H_2 O$
The catalysts $A, B, C$ are respectively:

Which of the following drying agents reacts with $CO_2$ and also removes moisture?

In $para-hydrogen$,what is the nature of the electron spin?

How do ortho and para hydrogen differ from each other?