已知f(t)的波形如下图所示，则f(t)的表达式为Af(t)=tε(t)tε(t)Bf(t)=tε(t)(t)ε(t)Cf(t)=(t)ε(t)(t)ε(t)Df(t)=(t+)ε(t)(t+)ε(t+)