Researchers conditioned a group of flies to associate a particular odor with a weak electric shock. Twenty-four and forty-eight hours later the researchers conducted tests on the flies, both individually and in groups, to determine whether the flies retained the conditioning. When tested individually, the flies were significantly less likely to avoid areas marked with the odor. The researchers hypothesized that in the presence of the odor, a fly that retains the conditioned association gives off an alarm signal that arouses the attention of any surrounding flies, retriggering the association in them and thereby causing them to avoid the odor.

The researchers' hypothesis requires which of the following assumptions?

A:The flies do not give off odors as alarm signals.

B:Flies that did not avoid the odor when tested individually were not merely following other flies' movements when tested in a group.

C:Flies that did not avoid the odor when tested individually were less likely than the other flies to avoid the odor when tested in a group.

D:Prior to their conditioning, the flies would likely have found the odor used in the experiment to be pleasant.

E:An electric shock was used during the flies' conditioning and during the later tests.