Hi, I'm preparing for my Base Programming Exam and I'm stuck with one question from the Pass4Sure catalogue. The explanation doesn't really help: The right answer is A. While I do understand why A doesn't produce a missing value in the last observation, I don't get why any of the other options should. I've pondered over this one for a while now but I'm not getting further. Especially option D should effectively do the same as A in my opinion. I would appreciate your help!
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