单项选择题
设f(x)连续，则
=______．
A．
f(t)(t-x)dt
B．
f(x)(x-t)dx
C．
f(t)(x-t)dt
D．
f(t)(t-x)dx
f(t)(t-x)dt B．
f(x)(x-t)dxC．
f(t)(x-t)dt D．
f(t)(t-x)dx