Laura bought a whole carton of milk.  Peter drank~$\dfrac{2}{3}$~of it and his sister, Mary, drank~$\dfrac{4}{9}$~of it.  How much less of the carton of milk did Mary drink than Peter? 
选项为:A:$\frac29$  B:$\frac13$  C:$\frac23$  D:$\frac49$  E:$\frac59$  请回答其中一个选项.