relationship between the hydrodynamic and thermal boundary layers

Question

Water (Prandtl number = 6) flows over a flat plate which is heated over the entire length. Which one of the following relationship between the hydrodynamic boundary layer thickness(d) and thermal boundary layer thickness (d_t) is true                                       

      (a) $\delta_{t} =\delta$                                                              (b) $\delta_{t} < \delta$

      (c) $\delta_{t} > \delta$                                                             (d) can not be predicted