However, secondary admission requirements are typically set on the basis of preventing overenrollment (versus department capacity), rather than based on some threshold of solid understanding. Also, in theory, C grades were supposed to mean solid enough understanding to move on to following courses.
For example, at Texas A&M, computer science, computer engineering, and biomedical engineering aspirants need to aim for a 3.75 GPA to pass secondary admission. But biological (agricultural), chemical, industrial, materials nuclear, and petroleum engineering essentially admit everyone who passes their classes (down to 2.0 GPA). It is hard to believe that the latter majors depend that much less on a solid understanding of frosh level courses than the former majors.