After a test, teacher Valeria collects the data of the scores. Given that:  The average score of class $A$ is $76$.  The average score of class $B$ is $84$.  The average score of class $C$ is $89$.  The average score of class $A$ and $B$ is $79$.  The average score of all the three classes is $81$.  There are $40$ students in class $A.$ What is the ratio of number of students of class $B$ to that of $C$? 
选项为:A:$3:2$  B:$1:1$  C:$4:1$  D:$5:2$  E:$2:3$  请回答其中一个选项.