A:m2﹣4
B:m2﹣2m+4
C:m2﹣4m+4
D:m2+4m﹣4
在日常生活中,用10N的拉力一定不能提起重15N的物体的简单机械是( )
把正确答语前的字母填到横线上
①6How many birds do you see?____
②I'd like car 18,please.____
③What's this?____
④How many balls do you have?____
a. I have 11. b. OK c. My new crayons d. I see 15.
小丽用照相机对某同学进行拍照,拍出的底片如图甲所示,若相机的焦距不变,要使底片的像如图乙所示,则( )