A sophomore intern is terrible advice. Making connections in a dept and at a company you’d be interested in long-term is what an internship is all about.

That’s not to say you shouldn’t leverage your offer to try and get one from Firm B and then accept A if you haven’t heard back in time. They might rush for you, but keep in mind you’re a soph intern too so you’re not exactly at the top of the priority list. If Firm B comes through with an offer before or after it doesn’t matter. Go there and make an impression