68. A force of 10

A force of 10³ newton stretches the length of a hanging wire by 1 millimetre. The force required to stretch a wire of same material and length but having four times the diameter by 1 millimetre is

(a)   4×10³   N                                       (b)  16×10³  N

(c)  (1/4)×10³  N                                     (d)   (1/16)×10³ N

Answer-b

(3)