The secondary unbalanced force produced by the reciprocating parts of a certain cylinder of a given engine with crank radius r and connecting rod length l can be considered as equal to primary unbalanced force produced by the same weight having
A. an equivalent crank radius r2 /4l and rotating at twice the speed of the engine
B. r2 /4l as equivalent crank radius and rotating at engine speed
C. equivalent crank length of r2 /4l and rotating at engine speed