Given data:
ID = 0.025 m
L = 4 m
volumetric flow = 0.7 m^3/h
volume of the pipe = 3.14 x (0.025)^2/4 x4 = 1.962 x 10^-3
Mean residence time = 1.962 x 10^-3/0.7 = 2.803 x 10^-3 h
But, we have asked in sec, hence 2.803 x 10^-3 x 3600 = 10.09 sec
Finally the answer is A.
Thanking you