The harmonic mean of a set of non-zero numbers is the reciprocal of the average of the reciprocals of the numbers. What is the harmonic mean of $1$, $2$, and $4$? 
选项为:A:$\dfrac{3}{7}$  B:$\dfrac{7}{12}$  C:$\dfrac{12}{7}$  D:$\dfrac{7}{4}$  E:$\dfrac{7}{3}$  请回答其中一个选项.