People who aren't member of the armed forces.

The first seven Americans named as astronauts in 1959 were pilots from the Army, Air
Force, or Marines. The selection committee believed that their flying experience would be
valuable in space flight. Nowadays, though, many astronauts are civilians without military
experience but with relevant degrees in science, engineering, or math.

the purpose of the heading in the passage is to identify who is telling the story.

this excerpt is part of the book sugar changed the world (2010) written by marc aronson and marina budhos. as the book has been written by two people, the heading "marc" indicates that this specific excerpt is being narrated by aronson. in the fragment, marc describes his grandfather's story after hearing it from his cousin. however, there are other excerpts where budhos is the one that tells the story of her family.

aronson and budhos, who are husband and wife in real life, decided to write sugar changed the world when they found out how sugar, which had an important role in slavery and economy, changed their ancestors' lives.

b b b b b b   b b b   b b b b b

the answer is b. the tracer looks harmless in flight, but the pilot’s bitten lips and white knuckles suggest how dangerous it can be.