that 可以引导主语、宾语、表语、同位语、定语从句，在前四种从句中that 没有任何具体的含义，也不充当从句的任何成分，所以当从句意义和结构完整时，关联词往往选择that.
1._______ we will do next is still under discussion.
A. That B. When C. Where D. What
2.He found at last _______ he had been looking for.
A. that B. what C. where D. which
3.He was overcome by the fear _________ he had lung cancer.
A. which B. what C. that D. of
4.The doctor insists that my brother give up smoking.