(____) Is it on your desk?
(____) Sure.
(____) Excuse me! Can I use your eraser?
(____) Where is my eraser?
(____) Look! It's under the chair.
(____) No, it isn't.
① xx−2−1x2−4=1
② xx−1−1=3x+1 .
下列说法中正确的是 ( )