12. In a certain middle school, the number of boys and girls is the same. $\frac{3}{4}$ of the girls and $\frac{2}{3}$ of the boys go on a field trip. Then the ratio of the girls who go on the field trip to the total number of students who go on the field trip is ( ).
(A) $\frac{1}{2}$
(B) $\frac{9}{17}$
(C) $\frac{7}{13}$
(D) $\frac{2}{3}$
(E) $\frac{14}{15}$
12. B.

Let the number of boys be $a$. Then the number of girls is also $a$. Therefore, $\frac{\frac{3}{4} a}{\frac{3}{4} a+\frac{2}{3} a}=\frac{9}{17}$.