Final answer:
The intensity difference between stages can refer to multiple contexts, such as wave interference patterns or demographic stages. In the context of sound levels, an intensity that is twice as high usually corresponds to a 3 dB increase.
Step-by-step explanation:
The question pertains to changes in intensity levels in physics, specifically relating to wave phenomena, sound levels, and population growth stages in demographic transition. However, the question seems to conflate different scientific concepts, making it unclear which exact stages are being referred to. For clarity, answers to two potential interpretations will be provided.
In one interpretation, the question may relate to the intensity and maximums in a wave interference pattern. In such a case, the intensity difference between the 3rd and 4th maximums would depend on the specifics of the wave interaction, and without additional information regarding amplitudes or phase differences, a definite answer cannot be provided.
In another interpretation that relates to demographic transition stages, transitioning from Stage 3 to Stage 4 involves changes in birth rates and death rates. Option (b) Birth rates and death rates decline in Stage 4 compared to Stage 3 is correct, as a decline in both would be necessary for such a transition, leading to a change in population growth rates.
When discussing sound levels, intensity differences in decibels can be calculated using the logarithmic relation of the decibel scale. An intensity that is twice as high as another typically represents an increase of about 3 dB.