Grey class

Drilling work

(10⁵ m)

Construction time

(10³)

investment

(RMB 10⁶)

Gas control cost

(Yuan/t)

Prevention of secondary disasters (%)

Best (I)

7.5

8.9

15.9

33.2

126

Better (II)

15

17.8

31.8

66.4

84

Bad (III)

22.5

26.7

47.7

99.6

42