由染色体的变化引起生物特征改变的变化是( )。
A．DNA突变
B．RNA突变
C．染色体变异
D．基因重组、基因突变和染色体变异
参考答案：D
— How is Emmy?
— I have no idea. She hasn't written to me ______ we met last time. [ ]
A. since
B. while
C. as
D. before
每个接触网工区要有安全等级不底于（）级的接触网工昼夜值班。
