What could be a theme of the following story? A group of friends uncover a secret tunnel that leads them to a world untouched by civilization. There they must battle creatures they have never seen and elements of nature.

The frontier

The hero

Society

The American dream

1. in which sentence is the underlined portion a predicate nominative? for astronauts, going into space to repair orbiting equipment becomes a challenge. for astronauts, going into space to repair orbiting equipment becomes tricky. for astronauts, going into space to repair orbiting equipment becomes challenging. for astronauts, going into space to repair orbiting equipment becomes frequent.