In the assignment dataset with more recent electricity bill data $(2022-2024),$ both the highlow method and regression analysis were less efficient predictors of the total cost equation than the same two methods were with the example data $(2019-2021).$

Choose the response that it the most logical explanation for why the predictive value has declined.

Select one: $$

a$.$ Changes in the method in which Delivery Charges, a category of costs on the bill but which have no cost detail, are calculated and charged.

b$.$ Changes in the monthly base charge and the energy charge $($i$.$e$.,$ the cost per kilowatt hour$).$

c$.$ The variance in the number of days included in the monthly billing cycles.

d$.$ Changes in the rate of sales tax.