A chemist is investigating an unknown gas. She observes that it combines with oxygen. It takes two liters of oxygen to fully react with one liter of the unknown gas, and that the resulting volume of gas is also two liters. What does she conclude?

A.) The unknown gas is diatomic.

B.) Oxygen is diatomic.

C.) The unknown material is beryllium.

D.) The unknown material is sodium

I think its's A or B please help!