Explain scope and life time of variables in C(Storage classes)

In C programming, the scope and lifetime of variables are determined by their storage classes. There are four types of storage classes in C:

1. Automatic Storage Class: Variables declared inside a function or a block have automatic storage class. The default value is garbage. The scope is local to the function within which they are defined. The lifetime is till the control remains within the block in which the variable is defined.

2. External Storage Class: Variables declared outside of all functions are said to have external storage class. The default value is zero. The scope is global, i.e., they can be accessed from any function. The lifetime is throughout the program.

3. Static Storage Class: Variables can be declared as static to increase their scope up to the file containing them. As a result, these variables can be accessed anywhere within a file. The default value is zero. The lifetime is throughout the program.

4. Register Storage Class: The register storage class is used to define local variables that should be stored in a register instead of RAM. This means that the variable has a maximum size equal to the register size (usually one word) and can't have the unary '&' operator applied to it (as it does not have a memory location). The default value is garbage. The scope is local to the block in which the variable is defined. The lifetime is till the control remains within the block in which the variable is defined.

In summary, the scope of a variable determines the portion of the code where a variable can be accessed. The lifetime of a variable determines the period during which the variable exists in the memory while the program is executing.