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.
