已知函数f(x)=(sinx-cosx)sinx，x∈R，则f(x)的最小正周期是（ ）。
π
Every time I went to see him I prepared some presents but every time he ignored them,
without giving me anything ____. [ ]
A. in cash
B. in relief
C. in charge
D. in return
牵引重车在前，空车在后的空重混编列车时，应如何操纵？
