Water $(C_p=4.18 \: kJ/kg.K)$ at $80^{\circ} C$ enters a counterflow heat exchanger with a mass flow rate of $0.5 \: kg/s$. Air ($C_p=1 \: kJ/kg.K$) enters at $30^{\circ} C$ with a mass flow rate of $2.09 \: kg/s$. If the effectiveness of heat exchanger is $0.8$, the LMTD (in $^{\circ} C$) is
- $40$
- $20$
- $10$
- $5$