Ms. Blackwell gives an exam to two classes. The mean of the scores of the students in the morning class is 84 , and the afternoon classs mean score is 70 . The ratio of the number of students in the morning class to the number of students in the afternoon class is \dfrac{3}{4}. What is the mean of the scores of all the students?
Answer Choices
A. 74
B. 75
C. 76
D. 77
E. 78