15. Express assumption of risk is when:

a. the parties agree beforehand that the defendant will relieve the plaintiff of his or her legal duty towards the defendant

b. the parties agree beforehand that the plaintiff will do everything in his power to protect the defendant from known risks

c. the parties agree beforehand that the defendant is only responsible for unknown risks

d. the parties agree beforehand that the plaintiff will relieve the defendant of his or her legal duty toward the plaintiff