a. You will have to go over your homework again because it's full of mistakes.

• "Go over" means to review or examine carefully.

c. The milk smells horrible. It must have gone off, so you will have to put lemon in your tea.

• "Go off" means to become rotten or spoiled, often used for food or drinks that have become unfit for consumption.

d. As two of the staff here have gone down with flu, we can't finish the work today.

• "Go down with" means to become ill with a particular sickness or disease.