Which of the following is the correct definition of a function to find the length of a list?

	(define (length l) (if (equal? l ()) 1 (+ 1 (length (cdr l)))))
	(define (length l) (if (equal? l ()) 0 (+ 1 (length (cdr l)))))
	(define (length l) (if (equal? l ()) 0 (+ 1 (cdr (length l)))))
	(define (length l) (if (equal? l ()) 0 (length (cdr (+ 1 l)))))
	(define (length l) (if (equal? l ()) 0 (length (+ 1 (cdr l)))))