If the Nusselt number $(\text{Nu})$ for heat transfer in a pipe varies with Reynolds number $(Re)$ as $\text{Nu}$ ${\displaystyle \propto }\; Re^{0.8}$, then for constant average velocity in the pipe, the heat transfer coefficient varies with the pipe diameter $D$ as

1. $D^{-1.8}$
2. $D^{-0.2}$
3. $D^{0.2}$
4. $D^{1.8}$