1.-What do you know about the world's population?

1.-What do you know about the world's population?
-I know China and India are the countries____more than one billion.
A.with a population of B.with populations
C.than have populations of D.which has a population of
2.Are you quite clear in your (mind) what you should do 括号内替换,下同
A.head B.idea C.experience D.thought
3.How time flies!No one can (call back yesterday).
A.get lost time back B.remember yesterday
C.keep yesterday back D.return to yesterday
4.The trallers(became trapped) in the forest in the bad weather.
A.were lost B.got into trouble
C.were in trouble D.got lost
Please tell me why?