In a middle-school mentoring program, a number of the sixth graders are paired with a ninth-grade student as a buddy. No ninth grader is assigned more than one sixth-grade buddy. If $\frac{1}{3}$ of all the ninth graders are paired with $\frac{2}{5}$ of all the sixth graders, what fraction of the total number of sixth and ninth graders have a buddy? (2015 AMC 8 Problems, Question \#16) 
选项为:A:$\frac{2}{15}$  B:$\frac{4}{11}$  C:$\frac{11}{30}$  D:$\frac{3}{8}$  E:$\frac{11}{15}$  请回答其中一个选项.