1. A 21 lb piston slides in a lubricated pipe. The clearance between the piston and pipe is 0.001 in. What is the shear stress exerted on the piston by the fluid when the speed of the piston is 21 ft/s ? Assume the viscosity of the fluid is 1.36 × 10-4 lb·s/ft². A. 34 lb/ft² B. 17 lb/ft² C. 40 lb/ft² D. None of these 5 in 6 in