Let `f` be a non-negative function defined on the interval `[0,1]`. If `int_0^xsqrt(1-(f\'(t))^2)dt=int_0^xf(t)dt, 0lexle1` and `f(0)=0`, then (A) `f(1/2)lt1/2` and `f(1/3)gt1/3` (B) `f(1/2)gt1/2` and `f(1/3)gt1/3` (C) `f(1/2)lt1/2` and `f(1/3)lt1/3` (D) `f(1/2)gt1/2` and `f(1/3)lt1/3`