Why isn't this c program working?

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#include <stdio.h> #include <stdlib.h> #include <ctype.h> #include <string.h> #include <math.h> char a = 'a'; char b = 'F'; char c = '7'; printf("%c \n", toupper(a) ); printf("%c \n", toupper(b) ); printf("%c \n", toupper(c) ); return 0; }

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