Question

johannes wants to know whether soil, water, or both are needed for plant growth. he chooses to conduct experiments on a willow tree for five years to answer his question. how can his experimental methods be changed to improve his experiment?

he needs to ask scientists what they think.

he needs to conduct the experiment for a longer period.

he needs to use a larger sample size.

he needs to test more factors in the environment.

Explanation:

• Option 1: Asking scientists' opinions doesn't improve the experimental design or data collection, so it's incorrect.
• Option 2: The experiment is already 5 years long, and extending the time isn't the main issue here. The problem is about the sample size (using one willow tree), so this is incorrect.
• Option 3: Using only one willow tree (small sample size) can lead to unreliable results due to individual variability. Using a larger sample size (more plants) would make the experiment more valid and reliable, so this is correct.
• Option 4: The question is about soil and water for plant growth. Testing more environmental factors would complicate the experiment and deviate from the original question, so it's incorrect.

Answer:

C. He needs to use a larger sample size.