题目

Keeping a diary in English is one of the effective ways to improve our English writing ability. Compared with other forms of writing, it is shorter and takes  61  (little) time. It can help us to develop the habit of thinking in English. If we persist in this practice, gradually we’ll learn how  62  （express）ourselves in English. In keeping a diary in English, we certainly run up against many   63  (difficult). In the first place, it often happens that we have trouble in   64   (find) appropriate words and phrases to give expression to our mind. Secondly,  there  海龟中有连趾（ww）和分趾（WW、Ww）两种类型，开始时，连趾（w）和分趾（w）的基因频率为0.4和0.6。当海龟数量增加导致食物不足时，连趾的海龟更容易从海水中得到食物。若干万年后，基因频率变化成W为0.2，w为0.8。下列有关说法正确的是 [　　] A． 海龟基因频率的改变，是因为缺少食物的特殊环境使之产生了定向变异 B． 海龟受环境影响而产生的变异都是不能遗传的 C． 海龟基因频率的改变，说明海龟在自然选择的作用下发生了定向的进化 D． 海龟基因频率已发生了很大变化，说明海龟已经进化形成一个新的物种