A metal block has a mass of 750 g. 60% of its mass is magnesium, and the rest is copper. The density of magnesium is $1.7\,\text{g cm}^{-3}$. The density of copper is $9.0\,\text{g cm}^{-3}$. What is the density of the block?
- A$2.5\,\text{g cm}^{-3}$
- B$4.6\,\text{g cm}^{-3}$
- C$5.4\,\text{g cm}^{-3}$
- D$10.7\,\text{g cm}^{-3}$