Bobby and Sam decided to plant a vegetable garden. Sam had heard that earthworms help produce fertile soil. He suggested that they buy worms from the bait shop and add them to the soil before planting seeds. "This is like a science experiment" Bobby replied. If this were a science experiment, what would be the BEST testable question to guide the boys' research? A) What do earthworms eat? B) How do earthworms help plants grow? C) How does the number of earthworms in the soil affect plant growth? D) Does the presence of earthworms really change the soil they live in?