Since 1948, Unverferth Manufacturing has manufactured innovative agricultural equipment. In 2003, the firm started developing a new 12-row strip-till subsoiler, which prepares 10-inch-wide seed beds spaced 40 inches apart, a width that is commonly used in cotton production. Before introducing the new tiller, Unverferth developed and tested almost three dozen product prototypes. Unverferth used the ____ approach to innovation.