Which of the following is not an example of a structural element in uncle tom's cabin?

I haven’t finished the whole book, but based on the answers, I would guess C, since the other answers all seem to have something to do with the structure and development while C is more of an idea to take away from the book. I could be wrong, but those are my thoughts

Unmanned spacecraft are exploring the solar system more cheaply and effectively than astronauts are. Astronaut explorers can perform science in space that robots cannot. The National Aeronautics and Space Administration has a difficult task.

Explanation: