6. If the ratio of the degree measures of the three interior angles of a triangle is $3: 3: 4$, then the degree measure of the largest interior angle of this triangle is ( ).
(A) 18
(B) 36
(C) 60
(D) 72
(E) 90
6. D.
$$
180^{\circ} \times \frac{4}{3+3+4}=72^{\circ} .
$$