6. A teacher conducted a math test for the two classes he taught. Class A's average test score was 84 points, and Class B's average test score was 70 points, and the number of students in Class A is $\frac{3}{4}$ of the number of students in Class B. Then the average score of the students in both classes is ( ).
(A) 74
(B) 75
(C) 76
(D) 77
(E) 78