Consider the following worksheet of salaries, where some of the rows have been hidden. You want to find the average of the salaries for all employees who have been with the company for at least $15$ years. Which of the following is a valid way to calculate this average?

a$.$ Create the formula $=$IF$($B$4>=15,1,0)$ in cell D$4,$ copy it down column D$,$ and then calculate the average in some unused cell with the formula $=$SUMPRODUCT$($B$4$:B$44,$C$4$:C$44,$D$4$:D$44)/$SUM$($D$4$:D$44).$

b$.$ Create the formula $=$IF$($B$4>=15,1,0)$ in cell D$4,$ copy it down column D$,$ and then calculate the average in some unused cell with the formula $=$SUMPRODUCT$($C$4$:C$44,$D$4$:D$44)/41.$

c$.$ Create the formula $=$IF$($B$4>=15,1,0)$ in cell D$4,$ copy it down column D$,$ and then calculate the average in some unused cell with the formula $=$SUMPRODUCT$($C$4$:C$44,$D$4$:D$44)/$SUM$($D$4$:D$44).$

d$.$ All the these choices will produce the correct average.