Seven girls and three boys are standing in a line randomly. What is the probability that three boys are next to each other? 
选项为:A:$\dfrac{1}{15}$  B:$\dfrac{2}{15}$  C:$\dfrac{1}{5}$  D:$\dfrac{4}{15}$  E:$\dfrac{1}{3}$  请回答其中一个选项.