Dan went to Doctor to have an x-ray. Dan did not sign a written contract, and Dan and Doctor did not make an oral agreement regarding the x-ray. When Doctor billed Dan $500 for the x-ray, Dan refused to pay. Doctor sued Dan to recover the $500. Which of the following is true about Doctor's lawsuit?
a) Doctor can recover under the quasi-contract theory of promissory estoppel.
b) Doctor can recover under an implied contract theory.
c) Doctor cannot recover because there was no express contract.
d) Doctor cannot recover because Dan did not give consideration for the bargain.