Why A is incorrect?

Hi @mprezzy, Thanks for the question! Note that this is a Must Be True type question, which means that the answer choice we pick has to logically follow from what sentences presented in the stimulus. First, let’s take a look at (A), which tells us that b ridges built before about 1907 were built without thorough mathematical analysis and so were unsafe for the public to use. The first part of (A) is completely true and does directly follow from the passage: we’re basically told this in the last few sentences of the stimulus. However, (A) goes on to say that bridges built before about 1907 were unsafe for the public to use. This means that ALL bridges built before 1907 were unsafe for the public to use. But we can see almost immediately how this is too extreme a statement. There clearly could be bridges that happened to be designed safely with the rules of thumb analysis. In fact, think of how unlikely it would be for no bridges to be designed safely, but for so many bridges to be used for (presumably) hundreds of thousands of years. While there certainly probably are other bridges that aren’t designed safely, it’s too much of a stretch to say that all bridges built before 1907 are unsafe for public use, and so we eliminate (A). (E), on the other hand, can be properly inferred from statements given to us in the passage. We know that the rules of thumb were insufficient to completely assure the safety of bridges under construction; after all, they clearly did not assure the safety of the Quebec Bridge in 1907. Hope this helps! Feel free to ask any other questions that you might have.

Thank you.

@mprezzy, let us know if you have any other questions!