_________ that causes heavy haze sweeping over china remains unknown to many scientists.

___ weather it is! Let’s go for a picnic!

Many people are not________the danger of their children’s bad habits, which lead to failing to make their dreams come true.

two cameras, I would prefer ________one, which is very easy for me to carry

Where’s your brother —He ________ the library with his friend. He’ll be back soon.