Which one of the following statements in the context of balancing in engines is correct?

(a) Magnitude of the primary unbalancing force is less than the secondary unbalancing force

(b) The primary unbalancing force attains its maximum value twice in one revolution of the crank

(c) The hammer blow in the locomotive engines occurs due to unbalanced force along the line of stroke of the piston

(d) The unbalanced force due to reciprocating masses varies in magnitude and direction.