A thermally insulated beaker contains $300\,\text{g}$ of water, and its initial temperature is $30\,^{\circ}\text{C}$. Water at $100\,^{\circ}\text{C}$ is then poured in until the temperature of the mixture becomes $50\,^{\circ}\text{C}$. The specific heat capacity of water is $4.2\,\text{J}\,\text{g}^{-1}\,^{\circ}\text{C}^{-1}$. What mass of water is added?
- A$60\,\text{g}$
- B$120\,\text{g}$
- C$180\,\text{g}$
- D$750\,\text{g}$