C. The air is thick, as if before a storm, and the night is so dark that Rainsford thinks he could sleep with his eyes open.
Explanation:
The air is thick, this sets the eerie feeling of the story´s setting, you can imagine the thick air surrounding the characters and how that affects the setting how that gets a suspensful feeling, then the author adds, the night is so dark that he could sleep with his eyes open, this gets the suspense needed to create an eerie and suspenfil mood in the story.
