10. The harmonic mean of a set of non-zero real numbers is the reciprocal of the average of the reciprocals of these numbers. Then the harmonic mean of $1, 2, 4$ is ( ).
(A) $\frac{3}{7}$
(B) $\frac{7}{12}$
(C) $\frac{12}{7}$
(D) $\frac{7}{4}$
(E) $\frac{7}{3}$
10. C.

From the problem, we know that the average of the reciprocals of the three numbers is $\frac{1}{3} \times\left(\frac{1}{1}+\frac{1}{2}+\frac{1}{4}\right)=\frac{7}{12}$. Therefore, the harmonic mean we are looking for is $\frac{12}{7}$.