Problem with prep question

[papertrail]

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!

[Kurt_Bremser · Accepted solution]

options B and D use a simple totquantity + quantity calculation; they cause no automatic retain, always reference a missing value (totquantity), therefore the result will be missing.

option C is a simple syntax error.

[papertrail]

Now I get it, thanks a lot!

[gamotte]

Answer D uses the "+" operator which produces a missing value if one of the operands is missing.
The 0 in the retain statement only provides an initialization which is overwritten by the operation.