David is doing an investigation about factors that affect plant growth, so he plants two young elm trees near each other in his garden. he gives the first elm tree a gallon of water every week, and he gives the second elm tree fertilizer once a month. he plans on measuring the trees' heights and trunk girths every week for the next year. how can david best improve his investigation? a. test only one independent variable at a time b. do not measure the trees growth at all until a full year has passed c. use cedar trees instead of elm trees for the investigation d. place both trees in a greenhouse instead of his garden