7.下列积分中等于零的是 () .-|||-A. (int )_(-1)^1dfrac (dx)({x)^3} B. (int )_(-pi )^ndfrac (xsin x)(1+{x)^2}dx C. (int )_(-dfrac {1)(2)}^dfrac (1{2)}|x|dx D. (int )_(-3)^3(x)^3cos xdx