I can tell you for certain that you won't learn C in 21 days, and anyone claiming they can teach it that quick are trying to con you.
After 21 days you might know the basic syntax rules. Of course, previous programming experience from other languages helps a lot. To learn how to write efficient C programs takes more time, however. And to learn how to write clean, maintainable, well-structured C programs take many years.
There are hundreds of pitfalls in C, mainly because the language allows so much bad syntax, and many people wrongly believe that if the language allows something, it can't be bad programming.
Apart from that, a quick way to determine if a course is bad is if you see that they aren't teaching the standard. If they aren't, the teacher is probably not experienced enough to teach. The easy way to see this is if a header file with < > symbols around it is used in any example, and it isn't one of the following standard ones:
<assert.h>
<complex.h>
<ctype.h>
<errno.h>
<fenv.h>
<float.h>
<inttypes.h>
<iso646.h>
<limits.h>
<locale.h>
<math.h>
<setjmp.h>
<signal.h>
<stdarg.h>
<stdbool.h>
<stddef.h>
<stdint.h>
<stdio.h>
<stdlib.h>
<string.h>
<tgmath.h>
<time.h>
<wchar.h>
<wctype.h>
At the sight of any other <name.h> file, flee from the course.
