问题补充：
His sister left home in 1998，and since.
A. had not been heard of
B. has not been heard of
C. had not heard of
D. has not heard of
答案：
A. had not been heard of
(答案→)B. has not been heard of
C. had not heard of
D. has not heard of
解析：首先观察四个选项的特点：从语态来看，两个选项使用了主动语态，两个选项使用了被动语态，优先考虑出题者是否想要考查被动语态。主语His sister与hear of之间是动宾关系，因此判定此题考查被动语态。结合句意“他的妹妹1998年离开家，然后就一直杳无音讯”以及时间标志词“since”可知，此处应使用现在完成时的被动语态。
