“Underground” is the only word in the English language _____ begins and ends with the letters “und”.

Someone is knocking at the door. Is it Ann?—It _____ be her. She is giving a performance at the theatre now.

They do some exercises ________ Friday morning. [ ]

______ of the students in my class plan to go abroad for ______ study in the future.

_____ weekdays, he often gets up _____ six o’clock. [ ]