Ans: D
$\begin{array}{cl}
& \text{length on the map} : \text{actual length} \\
= & 1 : 50\ 000 \\
= & 1\text{ cm} : 50\ 000\text{ cm} \\
= & 1\text{ cm} : 0.5\text{ km}
\end{array}$
Hence, we have
$\begin{array}{cl}
& \text{area on the map} : \text{actual area} \\
= & 1\text{ cm}^2 : 0.5^2\text{ km}^2 \\
= & 1\text{ cm}^2 : 0.25\text{ km}^2 \\
= & 1 \times 40\text{ cm}^2 :0.25\times 40\text{ km}^2 \\
= & 40\text{ cm}^2 : 10\text{ km}^2
\end{array}$
Therefore, the area of the airport on the map is $40\text{ cm}^2$.
