Question 22N.1A.SL.TZ0.21

The unified atomic mass unit, u, is a non-SI unit usually used by scientists to state atomic masses.

What is u?

A.  It is the mean of the masses of a proton and a neutron.

B.  It is the mean of the masses of protons and neutrons in all chemical elements.

C.  It is $\frac{1}{16}$ the mass of an ${}_8{}^{16}\text{O}$ atom.

D.  It is $\frac{1}{12}$ the mass of a ${}_6{}^{12}\text{C}$ atom.