C function Mux.

I have this function that's a multiplexer.

// Enumerated type for a single bit.
 typedef enum { FALSE=0, TRUE=1 } BIT;

    BIT multi(BIT A, BIT B, BIT C, BIT D, BIT S1, BIT S0)
 {
        if(S1== FALSE && S0 ==FALSE)
           return A;

        else if(S1==FALSE && S0==TRUE)            
           return B;

        else if (S1== TRUE && S0== FALSE)
           return C;

        else
           return D;

}

For multiplexer, S1,S0 as a two-bit binary number, indexing into A,B,C,D, in the order given.

so like S1==0 & S0==0 refers to A and S1==0 & S0==1 refers to B etc etc.

I feel my code is close or way off or it is correct I just need to fix how I am testing it in main in which I have...

 assert(multiplexer(FALSE, FALSE, FALSE, FALSE, FALSE, FALSE) == A);
 assert(multiplexer(TRUE, TRUE, TRUE, TRUE, TRUE, TRUE) == D);

please and thanks in advance