Everyone is here _____ Li Lei, because he is ill inhospital.

He broke his legs while ＿＿＿＿＿.

This chapter is so difficult to ______ that I shall have to read it again later.

I don’t know when my son back, but when he back, I’ll call you.

has an internal clock that regulates thebeating of our hearts, and the ______ of our breathing