$3$M is introducing a new product and would like to estimate demand. $3$M believes the product is unique and none of its competitors have anything similar on the market. The company would like a long$-$term forecast of demand so it can determine if it needs to expand its sales force. Due to these factors, $\_\_\_\_$ is the best forecasting method to use.

a$.$ the Delphi method

b$.$ a time$-$series technique

c$.$ a sales force composite

d$.$ regression analysis