A hollow sphere has a small hole in it. On lowering the sphere in a tank of water, it is observed that water enters into the hollow sphere at a depth of 40 cm below the surface. Surface tension of water is 7 × 10⁻² N/m. The diameter of the hole is:
a. 1/28 mm
b. 1/21 mm
c. 1/14 mm
d. 1/7 mm