【单选题】设Ω 1 :x 2 +y 2 +z 2 ≤R 2 ,Ω 2 :x 2 +y 2 +z 2 ≤R 2 ;x≥0;y≥0;z≥0.u=f(t)是(-∞,+∞)上的偶函数,且在(0,+∞)上严格单调增加,则 A. xf(x)dv=4 xf(x)dv B. f(x+z)dv=4 f(x+z)dv C. f(x+y)dv=4 f(x+y)dv D. f(xyz)dv=4 f(xyz)dv