Please chooose two complete sentences from the following four choices A, B, C and D.A:Dream of Red Mansion Is the best-known novels. B:The greatest danger that a species faces in a rapidly coevolving ecosystem. C:This problem can be effectively solved only in this way. D:The encouragement from the tutor had a great effect on him that he worked harder. 答案: This problem can be effectively solved only in this way.; The encouragement from the tutor had a great effect on him that he worked harder.