The ratio of strength of a hollow shaft to that of a solid shaft subjected to torsion if both are of the same material and of the same outer diameters, the inner diameter of hollow shaft being half of the outer diameter is
a. 15/16
b. 16/15
c. 7/8
d. 8/7
a. 15/16
b. 16/15
c. 7/8
d. 8/7