Main argument is that: people in past likely read much more Because they had very few diversions to amuse them in comparison to today.
A - eliminated because argument doesn't mention anything about quality of books effecting the reading.
C - just because more books are sold today, doesn't mean that people today read more. Population today is much more than it was 100 years ago, so this point is refuted.
D - eliminated because the author doesn't mention 'cost of reading' being a valid cause. This is about having a diversion or amusement, not related to the cost of the diversion
E - even if horse racing was popular back then so what? That is just one diversion of the past, but we don't know whether there were many more diversions then or not. Eliminated
Only B seems correct, because if people back then had considerably less leisure time than we have today, then they were less likely to engage in their available diversions - whether those be reading or something else. Hence
B answer