It puzzles the scientists _____ some mammals produce their young _____ others lay eggs. 

You should not go rafting ________ you know how to swim.

—Why didn’t you tell him about the meeting?—He rushed out of the room _____ I could say a word.

(2013·莱州一中期末检测)He didn't expect that his parents could manage to know ________ is going on in his college life on the Internet.

What an unforgettable experience! I’ll write it down _______ it is still fresh in my memory.