Top growth of plant is normally satisfactory as long as the oxygen diffusion rate remains above-

(A) 20-10 power -8  g/cm square/minute 

(B) 30-40 10  power -8  g/cm square/minute 

(C) 10-20 10 power -8  g/cm square/minute 

(D) 20-80 10  power -8  g/cm square/minute