Four different-coloured bottles (red, yellow, blue, and green) are standing on a shelf in a line from left to right (not necessarily in the above order). One of those bottles is just about to fall off the right-hand edge of the shelf. The red bottle is between the yellow and blue bottles. The blue bottle is between the green and red bottles. The green bottle is not about to fall.

a) Which bottle is just about to fall?

b) How many possible orderings are there for the bottles, and what are they?

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