Answer:
A. a tunnel for a high speed train underneath the Alps
Step-by-step explanation:
- A 57 km long tunnel has been made form the Gotthard base that provides a high-speed rail link between the Swiss Alps and the northern and southern Europe.
- Thus it has revolutionized Europe and freight transport. And is the world's longest tunnel and has an undersea segment and is second deepest and the longest tunnel and was opened in 2016.