Shall we go hiking this Sunday? －Sorry, I _______. I’ll have to take a piano lesson.

My father _________to South Hill．He______ there for 5 days．

The air in Beijing is getting much ______ now than a few years ago.

They still can’t decide ________ with our difficulties in learning math.

We ____ the last bus and didn’t have any money for taxi, so we had to walk home.