Why are decomposition reactions called the opposite of combination reactions? Explain with the help of equations.

In the chemical formula of rust ${\mathrm{Fe}}_2{\mathrm{O}}_3.\mathrm{Y}$

The value of Y in the formula can be-

A. Y = $5{\mathrm{H}}_2\mathrm{O}$

B. Y = ${\mathrm{xH}}_2\mathrm{O}$

C. Y = $10{\mathrm{H}}_2\mathrm{O}$

"Enter your correct answer as A, B or C."

Identify the oxidizing and reducing agent in the following reaction.
${\mathrm{H}}_2\mathrm{O}+{\mathrm{Cl}}_2\to 2\mathrm{H}\mathrm{C}\mathrm{l}+\mathrm{S}$

The reaction in this question given in the book seems to have error and the modified question should be as given below.

Identify the oxidizing and reducing agent in the following reaction.

${\mathrm{H}}_2\mathrm{S}+{\mathrm{Cl}}_2\to 2\mathrm{HCl}+\mathrm{S}$