When Gregor Mendel discovered inherited traits, one trait he looked at was the height of bean plants. The offspring of two tall bean plants was likely to be tall itself. Robert tries to reproduce Mendel's experiment, but he finds no relationship between the heights of the plants he grows and the heights of their parent plants.

Which of the following factors probably affected Robert's results?

A.
the amount of water and sunlight each plant received
B.
the materials the plants' pots were made of
C.
the shape and size of the bean plants' leaves
D.
the type of bean plant