What would be the best title for this passage? Why are "blue polyethylene bags" mentioned in paragraph 2?

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问题 What would be the best title for this passage?
Why are "blue polyethylene bags" mentioned in paragraph 2?

选项 A、They are used to prop up bananas.
B、They protect growing bananas and speed ripening.
C、They help increase heat and humidity and so prevent injury.
D、Bunches of bananas are always covered with them.