In a recent survey conducted in a city, it was found that among three hundred houses which had burglar alarms, a burglary attempt was made only in one house, during the last three years. This clearly proves that fitting houses with burglar alarms is an effective deterrent to burglary.
Which among the following, if true, exposes the most serious flaw in the above conclusion?
(A) In the houses which had manual security during the day, the burglar alarms were activated only during nights.
(B) The bugler alarms were fitted in such a manner that their presence was not noticeable to outsiders.
(C) The burglaries were committed usually only during nights.
(D) The three year survey period saw a spurt in robberies and attempted robberies in the city.