Air samples collected from the intake and the return gates of a retreating longwall face show methane concentration values of 0.1% and 0.8% respectively. The production from the longwall face is 2000 tonne/day and the air quantity circulating the face is 15 m³/s. The rate of methane emission in m³ per tonne of coal produced is

A. 11

B. 9.5

C. 5.5

D. 4.5

Answer: Option D