Hello everyone! If you are a beginner, you could use site of Nazarbayev University (NU) to learn C language ---> http://sst-csci.com/csci151/
Everyone can register in this site and start to learn C language. There are training materials, quizzes, and other interesting things.

As I've mentioned in previous post, I am a beginner and in the 1st course. So our curriculum includes studying C, C++, Java, and Python languages. Now I will learn C language.

Some sites were suggested to learning C++ languages:
https://acmp.ru
https://informatics.mccme.ru
https://www.hackerrank.com
https://leetcode.com

You can also suggest sources of learning C nor C++, nor Java, or Python. It would be great help for me and other people, who study these languages)))

Thank you very much for attention! I hope this information will help you ^_^