If water becomes increasingly scarce in decades ______, water shortage will become a hot issue all over the world.
Jack is late again. It is of him to keep others waiting.
Jack, who pretended , heard, however, every word that was said.
The old man __________ in Tibet for two years as a volunteer teacher;it is an experience he often mentions.
Qian Xuesen tried many times _________ he found the solution to the puzzle about space technology.
Those the exam cheat must another exam tomorrow afternoon.