The hydrodynamic and thermal boundary layer thickness, then

Question

In the laminar flow of air (Pr = 0.7) over a heated plate, if $\delta$ and $\delta_{2}$denote, respectively the hydrodynamic and thermal boundary layer thickness, then                                                                                                   

      (a)  $\delta = \delta _{T}$                                                                      (b) $\delta > \delta _{T}$

      (c) $\delta < \delta _{T}$                                                                  (d) $\delta = 0 but \delta_{t}\neq 0$