The Mass of 6 Million Molecules of a Diatomic Gas $5.1 \times 10-19 \mathrm{~kg}$

The mass of 6 million molecules of a diatomic gas is $5.1 \times 10-19 \mathrm{~kg}$ . What is the atomic mass of the element in this gas?

A) $5.10 \mathrm{u}$
B) $2.55 \mathrm{u}$
C) $25.6 \mathrm{u}$
D) $51.2 \mathrm{u}$