The thermal boundary layer for laminar forced convection

Question

For the fluid flowing over a flat plate with Prandtle number greater than unity, the thermal boundary layer for laminar forced convection                                                          

      (a) is thinner than the hydrodynamic boundary layer

      (b) Has thickness equal to zero

      (c) is of same thickness as hydrodynamic boundary layer

      (d) is thicker than the hydrodynamic boundary layer