Referring to the Study in Item 2 Again, for What

Referring to the study in item 2 again, for what additional reason might the conclusion be unjustified?

A) There was too small a number of experimental variables being studied to permit an adequate analysis of relationships.
B) There was no adjustment for maturational processes.
C) The classes using the new method were all from one school which would allow the operation of extraneous variables.
D) There was no comparison group.