In this case, mentally set compound B aside for the moment. (a) Compound A reacts with an excess of compound B. You will see the word 'excess' used in this section and in the problems. Figuring out which substance is the limiting reagent is an area that many students struggle with.
I certainly hope it is something you pay attention to and remember. Just a bit below, I'm going to tell you (several times) how to determine the limiting reagent in a chemistry problem. Limiting Reagent Examples Limiting Reagent Problems #1-10 Problems solved using dimensional analysis only Limiting Reagent Problems #11-20 Stoichiometry Menu ChemTeam: Stoichiometry: Limiting Reagent Examples