题目

The bedroom door opened and a light went on, signaling an end to nap time．The toddle（初学走路的婴儿）, sleepy-eyed, clambered to a swinging stand in his crib．He smiled, reached out to his father, and uttered what is fast becoming the cry of his generation: iPhone!        Just as adults have a hard time putting down their iPhones, so the device is now the Toy of Choice for many 1-, 2- and 3-year-olds．The phenomenon is attracting the attention and concern of some childhood development specialists． Natasha Sykes, a mother of two in Atlanta, remembers the first time 假如由n个碱基组成的某基因，控制合成只有一条多肽链的蛋白质。若氨基酸的平均分子量为a，则该蛋白质的分子量为（　　） A．na/6　　　　　　　　　　　　　　　　 B．na/3－18（n/3－1） C．na－18（n－1）　　　　　　　　　　　 D．na/6－18（n/6－1）