Did this kid just get excited by a coincidence? Or is there something deeper going on here?

(Thanks Tina!)

The question asked for the range of the function SQRT(x). What are your thoughts about the way the student answered the question. What does it show about what this particular student knows.*

* *In the past, I’ve publicly kvetched about how the only questions that we grapple with on this blog are about the particularities, rather than the generalities, of student work. This is a time when I think that the most interesting questions are accessed through thinking about what this particular kid was thinking. I’d also be interested in hearing how you think this represents a trend in students’ thinking, in general.*

Thanks to Tina for the submission!

Marshall Thompson is offering a substantial bounty for the correct explanation for this mistake. (He knows the truth from talking to the student.)

Who’s got it?

Bob Lochel writes:

“I used this question as part of a benchmark assessment given to over 1,100 students at our high school, as preparation for the state test. Only 14% of students gave the correct answer B, while 66% of students chose A as their response. I’m not surprised that students would perform weakly on a domain/range question, but I am surprised that so many chose A. I featured this problem as a set-up for a domain and range activity on my blog: http://mathcoachblog.wordpress.com/2013/01/22/home-on-the-range-and-the-domain/, but feel free to share it with the mathmistakes crowd.”

So, what do we think about choosing A? Any theories?