The boundary layer flow over a flat plate with zero pressure gradients is shown in Figure 44.The control volume is defined as dashed line a-b-c-d and the depth is defined as w. Ahead ofthe plate the velocity is uniform, U. The velocity distribution across cd is approximated by   , here   denotes the boundary layer thickness. Which of thefollowing statements is correct?  
(A) the mass flux across bc equals  ,
(B) the mass flux across ab equals   ,
(C) the mass flux across cd equals  ,
(D) the mass flux across bc equals