D is the distance by which a surface would have to be moved in the direction ________ (1) to its normal vector towards

[answerhappygod]

D is the distance by which a surface would have to be moved in
the direction ________ (1) to its normal vector towards the
reference plane in _____ (2) fluid stream of velocity
u0 to give the same mass flow rate as occurs
between the surface and the reference plane in a real fluid.
A. 1. parallel, 2. a viscous
B. 1. streamwise, 2. an inviscid
C. 1. streamwise, 2. a viscous
D. 1. parallel, 2. an inviscid