The police can’t decide whether the suspect is guilty because he could prove he was________at the time of the crime.
Every year, we plant many trees on _______ side of the road to make our city more beautiful.
If there are ______ trees, the air in our city will be _______ cleaner.
You shouldn't ____ the boy's mistakes.
______our teacher is ill, ______ he still comes to our class to teach us.
The teacher said that the earth ______ around the sun.