When chemicals react in a closed container, which must be true? A The mass both before and after the reaction is the same.
B. The mass both before and after the reaction is variable.
C. The same number of compounds is present both before and after the
reaction.

I think its B.

Explanation:

Its b, because if we take for example, solid and liquid, two different masses, it produces different mass so it's not A. It's not c because we wouldn't know if the compounds are same or not. So, I strongly recommend you to go with B.

these include the fossil fuels, oil, natural gas, and coal and nuclear energy. hope this ! ^.^

-ella