I disagree. The best math grades I got in my undergrad are Bs plus some C+ about 10+ years ago. I retook Calculus III and LA in summer 2012, and DE, Multivariable Calculus, PhD Micro I, Macro I and grad Prob and Stat I all five classes in fall 2012, and got all A and A+. You can as long as you work hard and know you are good at it.