Where may variables be defined in C?

Outside a function definition (global scope, from the point of definition downward in the source code). Inside a block before any statements other than variable declarations (local scope with respect to the block).

And also read

1. Do Global variables start out as zero? 
2. Does C have boolean variable type?
3. Where may variables be defined in C?
4. To what does the term storage class refer? What are auto, static, extern, volatile, const classes?
5. What does the typedef keyword do?
6. What is the difference between constants defined through #define and the constant keyword?
7. What are Trigraph characters?
8. How are floating point numbers stored? Whats the IEEE format?
9. When should a type cast be used?
10. Can structures be assigned to variables and passed to and from functions?