Concept:Heat causes metals to undergo thermal expansion.
Explanation:During a hot sunny day, the aluminum roofing sheets absorb heat from the sun.
This raises their temperature and causes them to expand.
Because the sheets are firmly fixed to the roof frame, the expansion creates internal stress.
The expanding sheets slide or rub against one another and against the nails or trusses.
This friction and sudden movement produce the crackling noise.
Thermal contraction would occur when the sheets cool down, not during heating.
Thermal equilibrium would mean no net heat flow, so no such noise would be generated.
Conduction describes heat transfer through the material, not the cause of the sound.
Therefore, the crackling is due to the expansion of the sheets.
Answer:D. expansion of the sheets