If you don't understand something, you may research, study, and talk to other people _______ you figure it out.
because
though
until
since
The population of China ________ 1.3 billion,and 80% of the population________farmers.
Every possible means ________ to prevent the air pollution,but the sky is still not very clear.
It is reported that 40 percent of the land in Brazil________covered with trees.
Either the judge or the lawyers________wrong on the case at that moment.
The poet and ______ pianist is going to give us a talk this afternoon.