坚毅的
重力
礼物
有罪的
gravity：n.①重力，地心引力；②严重；③严肃
例句：Arrows would continue to fly forward forever in a straight line were it not for gravity, which brings them down to earth. 句意：要不是重力使箭掉落地上，它将会一直沿直线往前飞行。
故本题选B。
