Question: In the Generic Authentication Architecture (GAA) key derivation function, what is the purpose of the `FC` component when constructing the input string `S`, and how is it formatted if it consists of two octets?
Option 1: `FC` distinguishes between different algorithm instances; if two octets, both are variable (`FC1||FC2` where both can vary).
Option 2: `FC` identifies the key derivation function type; if two octets, the first is fixed (`FC1 = 0xFF`) and the second varies (`FC2`).
Option 3: `FC` specifies the input parameter order; always a single octet with no defined structure.
Option 4: `FC` indicates the desired key length; if two octets, both define the length in hexadecimal.
Explanation: The correct answer is Option 2 because `FC` is used to distinguish between different instances of the algorithm. When `FC` consists of two octets, it follows a specific format where the first octet (`FC1`) is fixed as `0xFF`, and the second octet (`FC2`) is variable, allowing for differentiation between instances while maintaining a standardized prefix.