Answer:
[A] $500 million.
Step-by-step explanation:
We have been given that the city of Austin can build roads or light rail. If 10 miles of roads cost $1 million and 2 miles of light rail cost $10 million. We are asked to find the opportunity cost of 100 miles of light rail.
First of all, we will find cost of 1 mile of railroad.
![\text{Cost of 1 mile of light rail}=(\$10,000,000)/(2)](https://img.qammunity.org/2020/formulas/business/college/e6joxy6ne9jr394dn94gsbcthug5fdh69l.png)
![\text{Cost of 1 mile of light rail}=\$5,000,000](https://img.qammunity.org/2020/formulas/business/college/crpa2fxoq1uidi0k50vq5plwvu6upsp063.png)
![\text{Cost of 100 miles of light rail}=\$5,000,000* 100](https://img.qammunity.org/2020/formulas/business/college/dlh1f7tjruoyy16ip9bv6577imqmit67nv.png)
![\text{Cost of 100 miles of light rail}=\$500,000,000](https://img.qammunity.org/2020/formulas/business/college/5zzpf37dtxoqlvp36zbo2obxmlcckf37ck.png)
Therefore, the cost of 100 miles of light rail is $500 million.