"You can do all your program in C language."

yes, you can "do" all your program in 'C', but whether you can meet specific code-size and/or execution-speed constraints is another matter entirely.

Of course, if you are rubbish at Assembler programming you are unlikely to gain any advantage over a good 'C' programmer with a good, modern optimising compiler.

As I've said before (twice in as many days), every language has its own advantages & limitations; no single language can meet all constraints at all times and in all places!