Why is the "volatile" keyword important when declaring a variable that is modified by an interrupt handler in C for embedded systems?

A It tells the compiler not to optimize away or cache reads/writes to that variable, ensuring the main program always reads its current value, which may change unexpectedly due to the interrupt B The volatile keyword permanently deletes the variable after one use C It makes the variable run faster by storing it only in CPU registers permanently D Volatile variables cannot be modified by interrupt handlers at all