C is a case-sensitive language (it distinguishes between uppercase and lowercase letters). All of C's keywords are spelled completely in lowercase; online help contains a complete list of C keywords.
You can declare names in any combination of either case, but many programmers prefer to use lowercase for variable and function names, saving uppercase for declaring symbolic constants. (A “symbolic constant” is a descriptive name that represents a constant value. In VOLUME.C, PI is a symbolic constant.)