By saying "They seemed 'a priesthood, rather uneven in their merits but uniform in their bearing..." (Lines 3-4, Para. 4), the author means that______.

A．there is no fundamental difference between professors and priests though they differ in their merits
B．professors at the University of Pennsylvania used to wear black suits which made them look like priests
C．professors are a group of professionals that differ in their academic ability but behave in the same way
D．professors are like priests wearing the same kind of black gown but having different roles to play