请问一道英语四级题目 After( )seemed an endless wait,it was his turn to

After( )seemed an endless wait,it was his turn to step into the doctor's office.
A.what B.that C.it D.there
答案选A.
After作为一个介词,后面要加名词作介词宾语,what seemed an endless wait是一个介词宾语从句,可以起到名词的替代作用
that 在宾语从句中不可以放在介词后面,若选it则after后面直接跟的变成了一个完整的句子,不合语法,there 同样会使after后的部分变成一完整句子

选what。考察名词性从句，典型的介词宾语。如果用that的话必须在介词after和that之间加入the wait.这样构成一个定语从句。the wait   是泛指名词作先行词加定语从句。