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题目内容
（国家开放大学管理英语）
They since last night. They are about to finish the work.（考点：现在完成进行时）
选择一项：
A.
are cleaning the system
B.
have been cleaning the system
C.
had cleaned the system
选择一项：
A.
are cleaning the system
B.
have been cleaning the system
C.
had cleaned the system
参考答案
