Refer to the Graph Showing the Locomotor Activity of Drosophila

Refer to the graph showing the locomotor activity of Drosophila melanogaster flies that have been subjected to various gene-knockout experiments. What can be inferred about the normal function of gene B in the graph?

A) Gene B has little or no effect on locomotor activity.
B) Gene B interacts with gene A in controlling locomotor activity.
C) Gene B interacts with gene C in controlling locomotor activity.
D) Gene B is required for locomotor activity.
E) There is insufficient data to make an inference.