A:I heard
B:I have heard
C:did I hear
D:have I heard
就北半球而言,夏季气压和风带( )
如图,已知∠ABC=52°,∠ACB=60°,BO,CO分别是∠ABC和∠ACB的平分线,EF过点O,且平行于BC,求∠BOC的度数.