Natalie loves to run marathons. Her last pair of running shoes have completely worn out. She checks a couple of popular Instagram accounts of fellow runners to see what type of shoes they are wearing. Many of the posts that she sees are sponsored by sports companies, but she still trusts an everyday runner over a professional. Natalie is using what type of source for her information search?

a$.$ Nonmarketing$-$controlled information source

b$.$ Consumer$-$to$-$consumer reviews

c$.$ Marketing$-$controlled information source

d$.$ Internal information