问题
选择题
Although the causes of cancer_____, we do not yet have any practical way to prevent it.
A．are being uncovered
B．have been uncovering
C．are uncovering
D．have uncovered
答案
答案：A
因为句子的主语the causes of cancer与谓语uncover 是被动关系，因此我们使用被动结构，只有A 符合这一条件。
