The two correct answers for situations where observers A and B might measure different frequencies for the same vibrating object are:
1. **Observer A and Observer B are moving at different speeds relative to each other.** - This situation is a classic example of the Doppler effect, where the relative motion between the source of sound (or any wave) and the observer causes a shift in the observed frequency.
2. **Observer A is stationary and Observer B is moving.** - In this case, the moving observer (B) will perceive a different frequency due to their motion relative to the source of the sound, again due to the Doppler effect.
The other options (both observers stationary and at the same distance, or stationary but at different distances) would not cause a difference in frequency measurements.