A. All levels of demand, whether individual, aggregate, local, national, or international are subject to change.
B. At the same time, science and technology add new dimensions to products, their uses and the methods used to market them.
C. Aggregate demand fluctuates with changes in the level of business activity, GNP and national income.
D. The demand of individuals tend to vary with changing needs and rising income.

Sentence A gives the basic premise of what is being discussed in the passage i.e. that all demands are subject to change. Thus, sentence A is apt as the opening sentence of the paragraph. D further discusses the individual demands and C discusses aggregate demand. DC discuss the demands chronologically and hence ADC is the mandatory pair. B is driving the paragraph on a tangential nuanced idea apart from demand. Thus ADCB is the right choice.