select the one best answer from the following

A$)$ demand fluctuations that depend on time of the year $,$week or day are called seasonality

B$)$the seasonal index is an estimate of how much the demand during the season will be above or below the average demand

C$)$seasonality allways occurs in summer $,$winter,spring and fall

D$)$ a and b are true

E$)$ b and c are true