Which part of speech is not appropriate in formal writing, but often used in daily conversation.?

How does this passage contribute to the overall meaning of the story

(1) fire extended humans’ geographical boundaries by allowing them to travel into regions that were previously too cold to explore. (2) it also kept predators away, allowing early humans to sleep securely. (3) fire, in fact, has been a significant factor in human development and progress in many ways. (4) other obvious benefits of fire are its uses in cooking and in hunting. (5) probably even more important, however, is that learning to control fire allowed people to change the very rhythm of their lives. (6) before fire, the human daily cycle coincided with the rising and setting of the sun. (7) with fire, though, humans gained time to think and talk about the day’s events and to prepare strategies for coping with tomorrow. the sentence that expresses the main idea is: (type the number of the sentence. then click “go.”)

Which revision, if any, best fixes the bolded (capitalized in this case) part of the sentence? female cardinals, a somewhat drab bird compared to the males, blend in well with their environment. no change somewhat drab birds a somewhat drab's bird somewhat drab bird's