Ⅰ.单句语法填空
1．Something must be done to prevent factories sending out ________ (poison) gases so as to keep the city from being polluted.
2．It was not until the early 18th century ________ the significance of the event was generally realized.
3．There are so many green trees and beautiful flowers everywhere. ________ is no wonder that the air here is so fresh.
4．The fog was so heavy this morning that drivers could hardly make ________ the things just ten meters away.
5．There has been the ________ (require) that every child__________ (give) the opportunity to be educated.
