Using the following information, prepare a vertical analysis of $2$ years of income statements. Fees earned is $$151,500$ for Year $2$ and $$148,600$ for Year $1.$ Operating expenses are $$121,200$ for Year $2$ and $$126,310$ for Year $1.$ Which of the following statements is true?

a$.$ Operating expenses have remained constant as a percent of revenue.

b$.$ Operating income has decreased as a percent of revenue.

c$.$ Operating income has increased as a percent of revenue.

d$.$ Operating expenses have increased as a percent of revenue.