A:hadn’t been heard of
B:hasn’t been heard of
C:hasn’t heard of
D:hadn’t heard of
设G为△ABC的重心, AG→ =2 AM→ ,则( )
王大爷靠墙用竹子围成了一个长方形篱笆(如图),已知这个长方形长20米,宽12米,求这个篱笆有____长?