A.R≈0.07ρ B.R≈0.045ρ C.R≈0.28ρ D.R≈0.1ρ
A.0.80~0.85 B.0.70~0.80 C.0.70~0.75 D.0.65~0.70
A.ρ≥300 B.ρ≥1500 C.ρ≥1000 D.ρ≥3000