—Are you free after school? —Sorry. I’ve planned to treat a friend of mine to dinner ________for his help.
Why the explosion occurred was ____ the laboratory attendant had been careless.
________ one foot before the other, slowly and purposefully, the deer walked up to_______ I was _______.
This field of scientific research remains________.
The teacher just took down the names of the students _______ during the exam.
______ to get a seat for the concert, he didn’t mind standing in a queue all night.