one sample on one rainy day. one sample a day on several rainy days. several separate samples on one rainy day. several separate samples on several rainy days.
I gave my students this multiple choice question: Betsy wants to find out if the rainfall in her town contains pollutants. The best way for Betsy to gather this information would be to collect

Only one answer is correct among these 4 choices:
- one sample on one rainy day.
- one sample a day on several rainy days.
- several separate samples on one rainy day.
- several separate samples on several rainy days.

Could you tell me which one is correct?

several separate samples on several rainy days.