—________ will you ask for help when you get into trouble?
—My parents, I think.
A:Who
B:What
C:Where
要测量河两岸相对的两点A、B的距离,先在AB的垂线BF上取两点C、D,使CD=BC,再定出BF的垂线DE,使A、C、E在同一条直线上,如图,可以得到△EDC≌△ABC,所以ED=AB,因此测得ED的长就是AB的长,判定△EDC≌△ABC的理由是( )
若直线ax+2y+1=0与直线x+y﹣2=0互相垂直,则实数a=( )