Ned is designing an experiment to test which hand sanitizer kills the most e. coli bacteria. in order for ned's results to be valid, what must he do? a. test each hand sanitizer on different days and at different times of the day b. test the first brand of hand sanitizer on e. coli bacteria, then test the second brand of hand sanitizer on a sterilized petri dish c. test the hand sanitizers first on e. coli bacteria and then on other bacterial species d. test the hand sanitizers on bacterial colonies that are kept in the exact same environmental conditions, such as temperature and light level