A

Script:

Sarah: You look tired, Henry. What did you do last night?

Henry: Hi, Sarah. I watched the new season of Our Frontier. I didn’t stop until my mother told me to go to sleep at midnight.

Sarah: What is the show about?

Henry: It’s about human life on Mars with aliens. Do you believe aliens exist?

Sarah: We have many achievements in space science and exploration. If there were aliens, humans would have found them by now.

Henry: Well, I don't think we're the only intelligent life in the galaxy. In 2004, a US Navy commander said he saw a UFO when he was flying a plane. This claim was later supported by a video of the event.

Sarah: I won't change my view until I see concrete proof of extra-terrestrial life.

Henry: That’s why we’re trying to go to Mars, the most likely habitable planet after Earth.

What does the word it in line 3 mean?