Learn c first then you can easily understand any other language...
While all turing complete languages can be proven to have equal computational abilities, it is near-sided to suggest learning one language prepares you for learning any other language.
When comparing languages, there are many aspects to take into consideration: paradigm, syntax, semantics, type systems, scope systems.
Had you said, learning a language like C will prepare you for leaning other languages of the imperative paradigm, I would have been more likely to agree. Still, I do not think this would be the case.
Take smalltalk as a counter example. Smalltalk introduced object oriented programming which is a concept you are not exposed to in C. In addition, C does not prepare you for the logical paradigm you would encounter with a language like Prolog. And what about native linked lists, homoiconicity, tail call recursion, monads, currying, lambdas, lisp macros, and list comprehensions with map reduces? You'd get exposed to all of these concepts in the functional paradigm, in a language like common lisp or Haskell, but it is unlikely you'd have a firm grasp of implementing these concept if you were limited to C.
It's possible that you feel learning other languages is easy because the languages you choose all have similar syntax and paradigm to C. For example, the assemblers, C++, C#, and Java are all imperative typed languages with infix syntax. If I threw you in front of a Haskell REPL for the first time, I am pretty confident you'd feel outside of your element.
That said, I strongly disagree with your perspective. If what you really meant to say is, "If you can learn C, you probably have the ability to learn any language", I might agree but this is entirely relative and subjective. It doesn't mean anything.
Finally, if you think learning C will prepare you for any language, I have a strong belief that C has restricted and limited your experience with other languages and has prevented you from learning key concepts in other languages which differentiate themselves from C.
For more on this, you should read Paul Graham's article on the "Blub Paradox". People think their language is the best because when they learn new languages, they use the new language like their old language and never learn the true power of the new tool set:
paulgraham. co m/avg. html (remove spaces)
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Also, in my humble opinion, it really doesn't make sense to ask which is better between C and C++. Better is a totally subjective and unquantifiable word. This question would be more practical if criteria was provided or goal metrics were suggested for success. By far a more educated question would be, "What are the differences between C and C++ or what are the advantages of one over the other"? The answer to the latter is very well answered on Quora:
quora. co m/What-are-the-advantages-of-using-C-over-C-1 (remove spaces)