I justwonder ________ that makes him so excited

How difficult physics is! I really want to drop it !—_________you come on，you will do it better.

Before a problem can be solved, it must be obvious _____ the problem itself is. [ ]

Would you like beef or chicken— _______. I am a vegetarian.( 素食者)

I shall never forget the day ________ Shen Zhou V was launched, ________ has a great effect on my life.