Assertion: The cities are dynamic in nature.
Reasoning: The functional classification of cities can change over a period of time.

Both Assertion (A) and reasoning (R) are correct and but R is not the correct explanation of A.

The cities are not static in their function. The functions change due to their dynamic nature. Even specialised cities, as they grow into metropolises become multifunctional wherein industry, business, administration, transport, etc., become important. The functions get so intertwined that the city can not be categorised in a particular functional class.