In this drawing, A does control 4 degrees of freedom, 2 rotation (rotation around X and rotation around Y) and 2 translation (X and Y origin). B only can control one degree of rotation (rotation around Z), C controls the only degree of freedom left which is a translation (Z origin).
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Since the features that are being evaluated are cylinders, this does not work. If B was a plan on the end of the tube, then yes it would work since A and B would contact the mating surfaces simultaneously, but this is not the case with this design. As far as the measuring software goes, PC-DMIS...
I would have to disagree with @greenimi. Datums A and B do not share a common axis. To use that type of call, they would have to share a common axis to constrain the required 3 degrees of freedom together being a primary datum.
