Why is it important to test only one variables during an experiment?

Asnail, elodea (aquatic plant), or both were added to the tubes and they were stoppered. tubes were placed under a grow light for 24 hours and the results recorded. the test tubes were then covered for 24 hours to produce a dark environment and the results recorded. which statement best explains the change in test tube d from the light setup to the dark setup.

Once a woman attains puberty, her overuse mature and release one egg per ovarian cycle. which hormone stimulates this event?

What is the next step in muscle contraction after the actin changes shape? (apex)