题目

完形填空       通读下面短文，掌握其大意，然后在各小题所给的四个选项中，选出一个最佳答案。       Every time we go into a museum, we are often told, “Don’t touch that!” The Please Touch Museum in Philadelphia is 1 the opposite. Here we are encouraged to 2 everything.My first visit to the museum was one year ago. I was 3 to visit the museum as soon as Mum told us its name.4 told us that we could climb and jump, and touch everything in the museum. This place is more like a 5 rather than a museum. It gives kids the chance to learn 5-溴尿嘧啶(Bu)是胸腺嘧啶(T)的结构类似物。在含有Bu的培养基中培养大肠杆菌，得到少数突变体大肠杆菌，突变型大肠杆菌中的碱基数目不变，但(A+T)/(C+G)的碱基比例略小于原大肠杆菌，这表明Bu诱发突变的机制是( 　)A．阻止碱基正常配对B．断裂DNA链中脱氧核糖与磷酸基连接C．诱发DNA链发生碱基种类置换D．诱发DNA链发生碱基序列变化