Generally schools can rescind an offer of admission to any student if their performance drops significantly. You should review the information you have from UCLA, especially the information that came with your admissions offer. Usually they'll say that certain performance (C or D possibly) will require an explanation to avoid losing the offer of admission. Still, as competitive as UCLA is, I would imagine that B's and C's would not cause them to drop you. Just be careful that you don't start out at that level when you get in your freshman year!