问题
选择题
He _______ more visits to his parents.He has been regretting not doing so since they passed away.
A．should pay
B．would pay
C．should have paid
D．would have paid
答案
C
此题考查的是should have done结构。该结构表达与过去事实不相符的语气，意为“本应该……”，而事实上未能做到。should含有责备、批评的意味。
，试计算时域均衡器输出y(t)的样值y0，y+1，y-1分别为多少？