问题补充:
The bridge is so important to the people here that once it must be carried out without delay.
A. designs
B. having designed
C. designing
D. designed
答案:
A. designs
B. having designed
C. designing
(答案→)D. designed
解析:这里用过去分词,相当于一个状语从句,完整的句子是: once if it is designed.