单项选择题

某点的应力状态如图所示,其主应力为______

A.σ1=30MPa,σ2=0,σ3=20MPa
B.σ1=20MPa,σ2=0,σ3=30MPa
C.σ1=30MPa,σ2=20MPa,σ3=0
D.σ1=30MPa,σ2=0,σ3=-20MPa