(C) weakens the argument by pointing out the unrepresentative sample, i.e. tests were not conducted in places such as western North America where the usual quality of light differs from that prevailing in Britain.
(D) weakens the argument by pointing out an alternate cause, i.e. that such glasses might be worn for fashion purposes in certain parts of the world.
(E) weakens the argument with counter evidence, i.e. other patients did not show this relationship between tinted glasses and depression or hypochondria.
Hope that helps! Please let us know if you have any other questions.
