Final answer:
The authors used a signal word in the passage to signal a problem that astronaut Jim Lovell and the Apollo 13 crew were facing, creating tension and urgency in the narrative.
Step-by-step explanation:
In the passage from The Race to Space: Countdown to Liftoff, the authors used a signal word to signal a problem. The use of signal words such as 'really bad' within the context of the Apollo 13 mission serves to alert the reader that the crew is facing a serious challenge. It illustrates the gravity of the situation that astronaut Jim Lovell and his colleagues had to confront, thereby adding tension and urgency to the narrative.