(B) is correct because the passage talks about a persistent belief that glass flows slowly downward due to its atomic structure thus explaining ripply windows in old houses, whereas the true cause of thicker bottom part of the windows is the glass manufacturing method (lines 43-45). This distinction is similar to the one described in (B) - belief that pottery/ window flaw is due to the material/ glass as opposed to the manufacturing process. (D) is incorrect because the passage never mentions the belief that modern windows are somehow inferior to custom-made ones.
