Complete the following statement: The atomic mass unit $($u$)$ is defined so that $1$ u is exactly equal to the mass of

$1/12$ of a carbon$-12$ atom.

$1/16$ of an oxygen$-16$ atom.

a single hydrogen atom.

$\backslash (1/4\backslash )$ of a helium molecule.

$1/32$ of an oxygen molecule.