If you have any programming experience, you can go through the entire Kernighan & Ritchie at a leisurely pace in about three to four afternoons. I can’t recommend that enough.
C is a really minimalist language: it is easy to learn, though quite difficult to use. On the other hand, C++ is the exact opposite, difficult to learn, but easy to use. I don’t like it, because it is so bloated, but I use it because I can get stuff done with it quickly.
The entire C language is practically a footnote in a book about C++.
C# is just an M$-Abomination®, made as an M$ response to Java’s popularity. I wouldn’t touch it wearing a hazmat suit.