D
Some psychologists claim that, in theory, the best way to understand another person would be through deep empathy, whereby one would gain a direct and complete grasp of that person' s motivations. But suppose they are right; then there would be no way at all to achieve understanding, since it is psychologically impossible to gain a direct and complete grasp of another person' s motivations. But obviously one can understand other people; thus these psychologists are wrong. The argument is most vulnerable to the criticism that it
A: fails to adequately define the key phrase "deep empathy"
B: assumes something that it later denies, resulting in a contradiction
C: accepts a claim on mere authority, without requiring sufficient justification
D: confuses a theoretically best way of accomplishing something with the only way of accomplishing it