The observed effect the author is trying to explain?
"The number of hospital emergency room visits by heroin users grew by more than 25 percent during the 1980s."
And the author's proposed cause? An increase in the use of heroin that decade.
The problem with (B) is that it does not account for the statistic that hospital emergency room visits by heroin users grew by more than 25 percent during the 1980s.
Compare that with (A), which not only accounts for the statistic but also does not support the author's conclusion that this increase is a result of more heroin use.
Hope that helps! Please let us know if you have any other questions.
