one lr questions , 3x

historians of north american architecture who have studied early nineteenth- century houses with wooden floors have observed that the boards used on teh floors of bigger houses were generally much narrower than those used on the floors of smaller houses. these historians have argued that, since the people for whom the bigger houses were built were generally richer than the people for whom the smaller houses were built, floors made out of narrow floorboards were probably once a status symbol, designed to proclaim the owner;s wealth.

which one of following , if true, most helps to strengthen the historians' argument?

a : in the early nineteenth century, a piece of narrow floorboard was not significantly less expensive than a piece of wide floorboard of the same length.

b: many of the biggest early nineteenth-century houses but very few small houses from that period had some floors that were made of materials that were considerably more expensive than wood, such as marble.

i chose b but a is the right choice. could someone tell me why? thanks

The stimulus is talking about some indications of "wooden floors" in early 19th century but not other materials. At this point, b is out of the scope.

Also for my understanding, (a) negates a statement twice (not...less expensive), which actually means a piece of narrow floorboard was "significantly more expensive" ("less expensive" means "cheaper", then "not... significantly cheaper" means "more expensive")

Feel free to drop your commonts on the above explanation. Thanks!