题目

如图是蛙的红细胞无丝分裂过程，和有丝分裂相比，下列叙述正确的是        A．分裂过程中没有出现纺锤体和染色体的变化        B．没有DNA和染色体的复制        C．分裂过程中细胞核缢裂成俩个细胞核，因此子细胞中染色体减少一半        D．无丝分裂只发生在原核生物的细胞分裂中，有丝分裂只发生在真核生物的细胞分裂中听下面一段材料, 回答第1至4题.1. How many American rivers are seriously polluted?A. Onesixth of them. B. Onethird of them. C. Half of them. 2. When did Susan start the work of cleaning the river?A. Three years ago. B. Last August. C. A few weeks ago. 3. What did the students find when they came to the river this year?A. There was no garbage left to clean up. B. The river had become clean that a lot of water birds came back. C. The river was much cleaner and they had to search for garbage. 4. What was the expected reaction of the local people into the river?A. Most of them would keep on throwing garbage into the river. B. They would join the students in changing the situation. C. They would become more aware of the pollution problem.