________ birds use their feathers for flight, some of their feathers are for other purposes.
A.Once B.If C.Although D.Because
（2015·福建） the students came from different countries, they got along quite well in the summer camp.
A. While B. Unless
C. Since D. Until
（2018·江苏）_______ you can sleep well, you will lose the ability to focus, plan and stay motivated after one or two nights.
A.Once B.Unless
C.If D.When
Tom is so independent that he never asks his parents’ opinion ________ he wants their support.
A.since B.once
C.unless D.after
（2016·天津）_____the average age of the population increases, there are more and more old people to care for.
A.Unless B.Until
C.As D.While
________ some people are motivated by a need for success, others are motivated by a fear of failure.
A.Because B.If
C.Unless D.While
