Two ideal gases, A and $B,$ are kept separately, under the same temperature $T$ and pressure $P.$ Gases A and $B$ have identical volumes.

When the two gases are mixed, a full chemical reaction occurs where only one product is formed$-$ideal gas $C.$ The volume of gas $C$ is the sum of the volumes of gases A and $B.$

Which of the following statements must be true regarding the molecular composition of the gases?

a$.$ Gases A and $B$ are mono$-$atomic molecules, gas $C$ is a di$-$atomic molecule

b$.$ Each one of the gases: A$,$ B and C is a di$-$atomic molecule

c$.$ Gases A and B are at least di$-$atomic molecules, gas C is at least tri$-$atomic molecule $(3$ atoms$)$

d$.$ It is not possible to determine without elemental analysis, and molecular weights

e$.$ Gases A and B are at least di$-$atomic molecules, gas C is at least tetra$-$atomic molecule $(4$ atoms$)$

Correct answer is b$,$ how to do this, totally have no idea