单项选择题

Research has shown that _________.

A.Dropouts are substantially below Ph.D.'s in financial attainment
B.the incentive factor is a minor one in regard to pursuing Ph.D. studies
C.The Ph.
D.candidate is likely to change his field of specialization if he drops outD. about one-third of those who start Ph.D. work do not complete the work to earn the degree