I just did this test a couple of days ago and I chose A for this question as well. However, now that I'm looking back at it, I agree with their answer. This is how I reasoned it: there are no/little evidence in the passage to support A, but there is explicit evidence in the passage to support B -->Line 6-12: "....brought to the forefront of our thinking an aspect of the constitution that is usually NEVER considered...". If a contested presidential race made people rethink about the current voting system, which is usually NEVER thought about, then that means a contested presidential race doesn't happen often, otherwise people would have questioned the voting system a long time ago. Therefore, B is correct.
For questions like this, it's all about choosing the BEST answer. It doesn't mean all the other choices are wrong, it's just that they are not as good as the correct answer.