Increasing friction is necessary for which of the following? A) free wheel of bicycle
B) Chain of bicycle
C) Bicycle handle bar
D) ball bearing in front wheel

The illuminance of a surface varies inversely with the square of its distance from the light source. if the illuminance of a surface is 120 lumens per square meter when its distance from a certain light source is 6 meters, by how many meters should the distance of the surface from the source be increased to reduce its illuminance to 30 lumens per square meter?