Hot oil at $110{ }^{\circ} \mathrm{C}$ heats water from $30^{\circ} \mathrm{C}$ to $70{ }^{\circ} \mathrm{C}$ in a counter-current double-pipe heat exchanger. The flow rates of water and oil are $50 \mathrm{~kg} \mathrm{~min}^{-1}$ and $100 \mathrm{~kg} \mathrm{~min}^{-1}$, respectively and their specific heat capacities are $4.2 \mathrm{~kJ} \mathrm{~kg}^{-1}{ }^{\circ} \mathrm{C}^{-1}$ and $2.0 \mathrm{~kJ} \mathrm{~kg}^{-1}{ }^{\circ} \mathrm{C}^{-1}$, respectively. Assume the heat exchanger is at steady state. If the overall heat transfer coefficient is $200 \mathrm{~W} \mathrm{~m}^{-2}{ }^{\circ} \mathrm{C}^{-1}$, the heat transfer area in $\mathrm{m}^{2}$ is
- $17.9$
- $1.1$
- $5.2$
- $35.2$