The annual sales, in thousands, along with three years of business$/$economic indices for the corresponding periods, are given below:

a$.$ Find out which index is most relevant to the annual sales.

b$.$ Determine the line of best fit for the best$-$fitting index.

c$.$ Do you recommend the inclusion of more than one index for forecasting? Why?

PERIOD $-1,2,3,4,5$

SALES $-8,18,22,14,12$

INDEX A $-101,103,103,112,116$

INDEX B $-98,107,112,103,101$

INDEX C $-110,116,104,101,107$