What is the main theme of george orwell's novel 'animal farm'?

Which sentence contains an allusion? a. pamela and her twin are as alike as two peas in a pod, and they do everything together. b. pamela has a weakness for chocolates, which she says she's had since early childhood. c. pamela's face was as white as a ghost when she learned the shocking news about her brother. d. pamela's backyard looks like the garden of eden, with dozens of green and lush plants. e. pamela found herself trapped between a rock and a hard place, and she didn't know what to do.