题目

Every year several thousand people are killed on the roads in Great Britain. Every year between one and two hundred thousand people are hurt. These people are killed or hurt in road accidents. There are rules to make the roads safe, but people do not always obey the rules. They are careless. If everybody obeys the rules, the roads will be much safer. How can we make the roads safer? Remember this rule. In Great Britain traffic keeps to the left. Cars and buses and bikes must all keep to the left side of the road. In most other countries traffic keeps to the right. Before crossing the roa某双链DNA分子共含有含氮碱基1400个，其中一条单链上(A+T)︰(C+G)=2︰5。问该DNA分子连续复制两次共需游离的胸腺嘧啶脱氧核苷酸的数目是                   （   ）    A．300个              B．400个            C．600个          D．1200个