Based on the Experiment in Which Drosophila Flies Were Raised

Based on the experiment in which Drosophila flies were raised at a higher than normal oxygen concentration, which of the following statements is most likely true?

A) Fly body size is constrained because there are not enough alleles that increase body size.
B) Fly body size is constrained because alleles that increase body size are selected against at normal oxygen levels.
C) At normal oxygen concentration levels, fly body size is under strong directional selection.
D) Fly body size would likely increase if flies were raised at lower than normal oxygen concentrations.
E) Increased oxygen levels appear to have little effect on fly body size, even after many generations.