【单选题】
It was____ then that I realized the importance of a good mastery of the language.
①
until
②
unless
③
not until
④
not unless
【单选题】
When we breathe, the oxygen in the air ____ with our blood and gives us life.
①
fills
②
goes
③
takes in
④
mixes
【单选题】
What（）next will be discussed at the next meeting.
①
to do
②
is doing
③
will do
④
shall we do