I took physics on from a B grade and really struggled (got a D overall), but admittedly I was very lazy at the beginning of the year and it probably had a knock-on effect. Really, if you're willing to work hard and iron out issues when they first arise, you may just be okay.
Critical Thinking doesn't take all that much time to prepare for, and is accepted as an extra subject by some universities. You basically deconstruct arguments and point out flaws, and begin to write better arguments in essays.