What does it mean for a student to guess an answer?

On my final in Aut Theory I wanted to ask a TRUE/FALSE/UNKNOWN TO SCIENCE question but did not want them to guess. Hence I had +4 for a right answer, -3 for a wrong answer. Here is the question: ------------------------------------------------------------------------------------------ For each of the following say if its TRUE, FALSE, or UNKNOWN TO SCIENCE. No Proof Required BUT you get +4 for every right answer and -3 for every wrong answer and 0 for an answer left blank. So                 DO NOT GUESS!!!!!!!!!!!!!!!!!!!!!!!!!!! a) If A is regular and F is a finite set then A UNION F is regular. b) If A is in P and F is a finite set then A UNION F is in P. c) If A is in NP and F is a finite set then A UNION F is in NP. d) If A is decidable and F is a finite set then A UNION F is decidable. e) If A is undecidable and F is a finite set then A UNION F is undecidable ------------------------------------------------------------------------------------------------- Note that they are all TRUE. A student who (as many did) answered T-T-T-T-F had the following conversation with me: BILL: You guessed! I told you DO NOT GUESS!!!!!!!!!!!! STUDENT: No. I REASONED that you would not make them all T, so by this reasoning the last one had to be F. I now see that my reasoning is wrong--- you would make them all T, but it was not guessing, it was reasoning.                 I claim the student was guessing, he claims he was not. What do you think? Having said that, the following IS a rational strategy: If I don't know the answer but it has nothing to with P vs NP then it has to be T or F. In this case guess since exp val is positive.  If the answer has to do with P vs NP then do not guess. This also raises a question- if they honestly thought (say) e was F I want to just give them 0,  whereas if they are guessing or using reasoning about `Bill wouldn't ...' I want to give them -3. But alas, we cannot read their minds or souls.