I was about to leave the office ______ an unexpected visitor came.A. while B. as C when D. before
Many things _______ impossible in the past are very common today.
The students in the university are all taking courses _______ a degree.
. _______ different kinds of pianos, the workers farther improved their quality.
A person _______ a foreign language must be able to use the foreign language _______ all about his own.
.The wallet _______ several days ago was found _______ in the dustbin outside the building.