Esther, a teenager, gets $$8$ per hour for babysitting. Steven lives two doors down and turns $15$ next month. Esther is worried that Steven may want to start earning extra money by babysitting, too, and may charge less money. This is a problem of

a bargaining power of suppliers.

b rivalry among existing firms.

c threat of new entrants.

d threat of substitutions.

e$.$ bargaining power of customers