So..what IS the answer??

There is some saying about 'you can lead a horse to water, but you can't make him drink' and I've decided the teaching equivalent is 'you can give a student the information and tools to learn, but you can't make him think'. I'm at my wits end with a couple of students. One of the topics in a course I teach deals with understanding the validity of resarch tools. I spend considerable time on different possibilities for demonstrating valildity in my lectures, I have student look at examples of how researchers provide this evidence, and I have multiple small group projects where students develop their own possibilities. There is also a book chapter on this topic which is supported with online learning center examples and practice provided by the textbook authors/publisher. The students that are pushing me over the edge at the moment are those students who refuse to think about this, rather they constantly ask "So, what IS the answer?" and do not seem to understand that there can be a variety of answers for a given situation depending on the context and usage of the research tool.
As a class activity, I divided the students into groups and gave them each a paper with a description of a measurement tool and had them devise ways to demonstrate the validity of the instrument. What the students didn't know was that I gave each group the same tool--the point was to show that people approach the situation differently and many correct answers are possible, it just depends on how the groups defined their context, etc. I even said this was the point of the exercise and still those same 'nonthinkers' asked "So, what IS the answer?".
Shouldn't graduate students be a little beyond this stage of intellectual development? Shouldn't they realize that not every situation has only 1 solution? Where did they get the idea that learning is about getting THE answer?