For laminar flow over a flat plate, the thickness of the boundary layer at a distance from the leading edge is found to be 5 mm. The thickness of the boundary layer at a downstream section, which is at twice the distance of the previous section from the leading edge, will be

A

10 mm

B

$$5 \sqrt 2 mm$$

C

$$\frac {5}{\sqrt 2} mm$$

D

2.5 mm