Best City Co$.$ opened cellular phone stores at nine locations in Manhattan to gauge consumer $s$ reactions to them. These stores, called Best City Mobile, carried more than $80$ cell phones from big manufacturers. Shoppers had the option to choose from seven mobile phone service carriers. Based on positive consumer reaction and the success of these stores, Best City opened stores nationwide. This is an example of

a$.$ product development

b$.$ test marketing

c$.$ project planning

d$.$ idea screening