Correct option is A
Total work completed by A, B and C = 2×3/2=3 days
Total work completed by A and B together = 100/30×3=10 days
Total work = 30 (LCM of 3 and 10)
Efficiency of C = 30/3-30/10=10-3=7 unit/ day
Required time = ( 14/3×30)/7=20 days