题目

      A recent study, published in last week’s Journal of the American Medical Association, offers a picture of how risky(冒险的) it is to get a lift from a teenage driver. Indeed, a 16-year-old driver with three or more passengers is three times as likely to have a deadly accident as a teenager driving alone, while the risk of death for drivers between 30 and 59 decreases with each additional passenger.     The authors also found that the death rates for teenage drivers increased sharply after 10 p.m., and especially after midnight. With passengers in the car, the drive下列有关“S”型曲线k值的改变与事实相符合的叙述是（   ）A．x轴为外界蔗糖液浓度，y轴为叶肉细胞渗透失水量，在c点时，去除细胞壁，k值将不改变B．x轴为光照强度，y轴为绿色植物实际光合作用量，在b点提高CO2的浓度，k值将不改变C．x轴为氧气分压，y轴为水稻根细胞对硅的吸收速率，在c点时中耕松土，k值将改变D．x轴为时间、y轴为某种群个体数，在b点改变环境条件或种群遗传因素，k值将不改