B Heather wants to go to the store with us.
An infinitive is a verb that hasn't been conjugated for a certain subject, so it sounds the same whether it's us, you, me, they, she or anyone doing it. (For example, while runs has an s in "she runs" and no s in "I run", with the infinitive, they are both the same: To run, as in "She wants to run" or :"I want to run")
Usually you can tell a verb is an infinitive if it has "to" in front of it.
