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 out

D.about one-third of those who start Ph. D. work do not complete the work to earn the degree

正确答案：about one-third of those who start Ph. D. work do not complete the work to earn the degree