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