Xavier opens a lemonade stand for two hours. He spends $$10$ for ingredients and sells $$60$ worth of lemonade. In the same two hours he could have mowed his neighbor's lawn for $$40.$ Xavier has an accounting profit of $q,$

a$.$ $$50,$$$10$

b$.$ $$90,$$$50$

c$.$ $$10,$$$50$

d$.$ $$50,$$$90$