The main reason that it's important to test only one variable at a time is so that

a. scientists can have many hypotheses tested at once.
b. quantitative data can be recorded.
c. qualitative data can be recorded.
d. scientists can easily tell which variable caused the results.

D. scientists can easily tell which variable caused the results.

Good luck!

when atoms of different types of elements join together, they make molecules called compounds. water consists of compound molecules made up of 2 hydrogen atoms and 1 oxygen atom. this is why it's called h2o. water will always have 2 times the number of hydrogen atoms as oxygen atoms.