Two blocks are connected together. One block has a density of $6.0\,\text{g cm}^{-3}$, while the other has a density of $9.0\,\text{g cm}^{-3}$. What is the combined density of the two blocks when they are joined together?
- A$7.0\,\text{g cm}^{-3}$
- B$7.5\,\text{g cm}^{-3}$
- C$8.0\,\text{g cm}^{-3}$
- D$15\,\text{g cm}^{-3}$