He expressed his gratitude to his head teacher, without _______-help he wouldn’t have made such great progress in his studies.

A．whose

B．his

C．whom

D．which

王刚新买了一套弹簧拉力器，内装5根相同的弹簧，说明书上写着：“把每一根弹簧拉长10cm需要10N的力”．王刚将其中4根并联装入拉力器，若王刚将弹簧拉力器拉长0.5m，则他的拉力是______．