Final answer:
The discrepancy between Joe and Shmo's observation that the bird is moving in different directions is due to them being in different reference frames. Hence, both observations can be true as motion is relative to the observer's position.
Step-by-step explanation:
When Joe looks at a bird and says it is moving to the right, and Shmo looks at the same bird at the same moment and says it is moving to the left, this is a clear indication that motion is relative to the observer's frame of reference. If they are observing the bird from different positions, what each one sees is true within their own frame of reference. Therefore, the answer to the question is A. Joe and Shmo must be in different reference frames. This is because direction and motion can appear differently depending on where the observer is located, similar to how the motion of boats A and B seem different to people on the respective boats as they pass by each other.
Using the example of the train platform, a boy on a moving train appears to be in motion to an observer on the platform, but from the boy's frame of reference on the train, he is not moving. The same principle applies to the bird and the observers: depending on their vantage points, their observations regarding the direction of the bird's movement can both be correct.