单项选择题
图示非周期信号u(t)的时域描述形式是：[注：l(t)是单位阶跃函数]
A．
B．u(t)=-l(t-1)+2·l(t-2)-l(t-3)V
C．u(t)=l(t-1)-l(t-2)V
D．u(t)=-l(t+1)+l(t+2)-l(t+3)V
图示非周期信号u(t)的时域描述形式是：[注：l(t)是单位阶跃函数]
A．
B．u(t)=-l(t-1)+2·l(t-2)-l(t-3)V
C．u(t)=l(t-1)-l(t-2)V
D．u(t)=-l(t+1)+l(t+2)-l(t+3)V