Engineered Fabrics makes bladders for airplane gas tanks. In the event of a puncture, the bladder will prevent the tank from exploding. The bladders must be defect-free. To check the bladders, company employees must be able to crawl in an 18-inch diameter hole in the bladder side and look for light leaks. A limitation on weight and height of individuals working in quality control at this company would be legal because it is an example of ____.