According to a report from $2019($pre$-$pandemic$),$ the two dominant "dollar store" retailers$$Dollar General and Dollar Tree$$were already rapidly expanding the number of stores in inner$-$cities and rural areas. They plan to open thousands more stores in these areas, which are markets they have traditionally served. The report's authors were concerned by this expansion. From their report:

By saturating communities with multiple outlets, dollar stores have made it much harder for full$-$service grocery stores to succeed, our report found, leaving neighborhoods with fewer jobs and reduced access to fresh, healthy food. Although most dollar stores sell only a limited selection of packaged foods, many people now rely on these chains for groceries.

Dollar Tree and Dollar General's expansion plans are an example of a$($n$)$: $\_\_\_\_\_$ marketing strategy.

a$.$

product development strategy

b$.$

diversification strategy

c$.$

market development strategy

d$.$

market penetration strategy

e$.$

product differentiation strategy