题目

人工诱变育种依据的遗传原理是（    ） A．染色体数目的变异           B．基因突变 C．基因重组                   D．染色体结构的变异 答案：BAll possible means ___ been tried to stop the air______A.has, pollutedB.have, pollutingC.have, being pollutedD.has, being polluted