During those years I also worked as a scientist exploring other parts of the world’s oceans. In 1973 and 1974 I joined expeditions that explored the Mid-Atlantic Ridge, an enormous underwater mountain range running down the middle of the Atlantic Ocean. This ridge is part of an even bigger undersea range that extends 40,000 miles around the world. In 1977, near the Galapagos Islands off the coast of Ecuador, we discovered enormous red worms, some of them more than eight feet long. These giant worms live inside white tubes and cluster around underwater hot springs. Which phrase from the excerpt best helps the reader understand what the Mid-Atlantic Ridge is?

other parts of the world’s oceans
part of an even bigger undersea range
enormous red worms
underwater hot springs