C is a simple language. Many people have quickly become proficient at it with only K&R to work from, and that is a thin book. The C language has many deficiencies but I don’t think many people would characterize it as difficult to learn among its peer programming languages. It has very few concepts that you need to learn.

C++, on the other hand, is exceedingly complex and a difficult language to become proficient in, though modern C++ is an improvement in that regard. Some of this is due to decades of legacy cruft, some is due to it being an atypically expressive systems language, all of which has been stirred together.