County building inspectors report that almost fifty percent of the homes ... ...

Which one of the following assumptions must be made in order for the author to be correct in drawing the conclusion stated in the passage?
(A) Thirty percent of the residential carbon monoxide gauges have been installed within the last fifteen years.
(B) ...
(C) ...
(D) ...
(E) ...

*This question is included in Nova Press: Set H - Even More Classification Practice