The flu is believed __________ by viruses that like to reproduce in the cells inside the human nose and throat.
Do you remember the chicken farm _____ we visited three months ago?
I was about to go to bed _____ one of my old classmates made a call to me.
After Yang Liwei succeeded in circling the earth, Chinese astronauts __________ walk in space.
Students are encouraged to _________ the design competition for robots at the university.
Is _______ some German friends visited last week?