HOW TO BECOME PROFICIENT IN C

So I claimed I was proficient in a number of languages on my resume, all of which I've barely used.

I think I can get away with some of it but I really think I need to know how to use C and Python well for the job. He said the job might require C# which is close enough to core C.

How do I become proficient with these languages in a week or so?

I refuse to read this thread.
However:
C# which is close enough to core C
You will never be hired by anyone, ever.  Not even for a non-programming job.