Plaintiff and Defendant went on a social outing to Defendant's fish camp. Defendant warned Plaintiff that she should not go onto an unfinished pier and that the boards at the end were not closely spaced. Plaintiff heard the warning but continued on without replying. After reaching the end of the pier, she started back, choosing her steps carefully. A loose board then gave way and she fell, tearing ligaments in her knee. The best argument by Defendant is:

$$ Contributory negligence.

Lack of proximate cause.

Assumption of risk.

The "last clear chance" doctrine.