The ratio of the number of students in two classrooms, `C_(1) and C_(2)`, is 2 : 3 . It is observed that after shifting ten students from ` C_(1) " to " C_(2)`, the ratio is 3 : 7. Further, how many students have to be shifted from ` C_(2)"to" C_(1)` for the new ratio to become 9 : 11 ?
A. 10
B. 15
C. 20
D. 8