这句句子的主语是“服药”这个动作,所以“持续”是“服药”的持续,是主动,所以要选C.即使改成人作主语也是主动也用C,因为“人服药”是主动,“人持续服药”也是主动：Patients taking this kind of medicine,if continuing,will surely be harmed by its side effect.服这种药的病人如果持续服用肯定会受到其副作用的伤害.
如果改成“药被服”作主语,还是整个动作在持续,还是主动,还是C：
This kind of medicine being taken,if continuing,will surely produce side effect.