Algae like fertilizer. Fertilizer is good for algae. Fertilizer improves the genetic traits of algae. Algal growth rate increases with fertilizer concentration.
I gave my students this multiple choice question: A student plans to grow algal colonies in solutions with different concentrations of liquid fertilizer. Which is a clear and testable hypothesis to guide this investigation?

Only one answer is correct among these 4 choices:
- Algae like fertilizer.
- Fertilizer is good for algae.
- Fertilizer improves the genetic traits of algae.
- Algal growth rate increases with fertilizer concentration.

Could you tell me which one is correct?

Algal growth rate increases with fertilizer concentration.