Practical dividing line between a ductile and brittle materials is suggested, when the ultimate elongation is about 5%. Generally, larger the knuckle radius, stronger is the corner torus section of a head. The knuckle radius provided should be less than ______________ of the head?

A. 6% of I.D.
B. 3 times the thickness
C. Either A. or B., whichever is larger
D. Either A. or B., whichever is smaller