The thickness of boundary layer at a section 4 m from the leading edge in mm will be

Question

A laminar boundary layer occurs over a flat plate at zero incidence of the flow. The thickness of boundary layer at a section 2m from the leading edge is 2 mm. The thickness of boundary layer at a section 4 m from the leading edge in mm will be                                             

(a) 2 x (2)²                   (b) 2 x (2)^1/2

(c) 2 x (2)^4/5                 (d) 2 x (2)^1/5