The issue with (C) is that it is unsupported by the passage. The passage merely tells us that a construction accident in early twentieth century resulted in bridge engineers no longer relying on "rules of thumb." The passage never tells us why engineers in 19th or earlier centuries relied on rules of thumb - is it because analytical methods were inadequate? Or was it because it saved time? Or was more cost effective? The bottom line is we simply have no information to determine the reason behind this practice, thus (C) would require us to make an inference that is outside the scope of the passage,
