单项选择题
A.2εS/d B.εS/d C.d/2εS D.d/εs
A.σ/3ε,σ/2εd B.σ/ε)eX,σd/ε C.σ/3ε,σ/3εd D.σ/2ε)eX,σd/2ε
A.σ/2ε0)eX B.σ/ε0 C.σ/ε0)x2 D.σ/ε0)eX