Question

a student set up an experiment to test the effects of soil compaction on plant growth. the student put equal weights of moist soil into 10 containers, planted a bean seed one inch deep in each container, and then firmly compacted the soil in 5 of the containers. after the seeds sprouted, the student measured the height of each plant every day and kept a record of the results. what else must the student do over time to ensure a valid experiment?

water any container in which the soil feels dry
water any of the plants that seem to be growing more slowly than the others
water the plants in compacted soil more than those in uncompacted soil
water all of the containers the same amount and at the same time

Explanation:

In a valid experiment, all variables except the independent variable (soil compaction here) must be controlled. Watering all containers equally and at the same time keeps water amount and timing constant, ensuring that any difference in plant growth is due to soil compaction, not watering differences. Other options introduce bias or change uncontrolled variables.

Answer:

D. Water all of the containers the same amount and at the same time (assuming the options are labeled A to D with the last option as D; if original labels differ, adjust. Here the last option is "Water all of the containers the same amount and at the same time")