Because they can LEGITIMATELY argue that two, none, or all of the answers are equally correct; and be right about the logic.
Yup--this is my DD, too. Yes, well, that's TRUE...I really see your point...and YET...what did they WANT you to pick? (I refrain from adding, "Think like a not-very-smart person...")
BTW, at least once this year, DH, DD, and I ALL picked the wrong answer on a reading comp question. Third grade, people. Lots of brainpower in the room. We puzzled over it together. All of us were quite mystified. I still don't know what the right answer was! (They all seemed wrong.)
