Two cells are joined in parallel, positive terminal to positive terminal. One has an emf of $2\ \text{V}$ and an internal resistance of $1\ \Omega$; the other has an emf of $4\ \text{V}$ and an internal resistance of $2\ \Omega$. An external resistor $R$ is connected across the combination, and its value can be chosen freely. The largest power that can be delivered to $R$ is closest to