A.
(define ( abs x ) (cond ((x>0) x) ((x=0 ) 0) ((x<0 ) (-x)) )) ;
B.
(define (abs x) (cond ((> x 0) x) ((== x 0) 0) ((< x 0) (-x))))
C.
(define ( abs x) (cond ((x>y) (-x)) ((x=y ) 0) ((x ;
D.
(define (f x y) (cond (( ((= x 0 ) 0) ((> x 0 ) (* y y)) )) 。