读句子,选答语。
A. She is my sister.
B. They are in the kitchen.
C. No, they aren't.
D. He's a student.
E. No, she isn't.
如图,AC、BD是平行四边形ABCD的对角线,AC与BD交于点O,AC=4,BD=5,BC=3,则△BOC的周长是____.
20世纪80年代以后,东北三省的工业发展滞缓,原因不正确的是( )