The displacement thickness at a section, for an air stream 
moving with a velocity of 10 m/s over a flat plate is 0.5 mm. What is the loss of mass rate of flow of air due to boundary layer formation in kg per meter width of plate per second?
(a) $6\times 10^{-3}$
(b) $6\times 10^{-5}$
(c)$3\times 10^{-3}$
(d)$2\times 10^{-3}$