to remove harmful substances to add color to increase its supply to decrease its temperature
I gave my students this multiple choice question: Water is often put through a treatment system before it is used in homes as drinking water. Which is the most likely reason water is treated before people drink it?

Only one answer is correct among these 4 choices:
- to remove harmful substances
- to add color
- to increase its supply
- to decrease its temperature

Could you tell me which one is correct?

to remove harmful substances