When we have trouble remembering what we've read, it's usually not _____.
because we're trying to cram too many things into our short term memory
because the ideas that we are trying to memorize are very hard
because memorizing key points is more difficult that memorizing large passages of text
none of these
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I'd go with B. The question messes me up for some reason because of the use of "not," so I decided to take out the word "not" and flip around a couple of things. B would translate to:
"When we have trouble remembering what we've read, it's usually because the ideas that we are trying to memorize are very easy"
And that just makes no sense