Was it two years ago _____ they first met? [ ]
A. since
B. which
C. while
D. that
答案:D
环境空气质量功能区的一类区执行()标准。
A.一级
B.二级
C.三级
D.Ⅰ级
显性致死试验的遗传学终点为()
A.基因突变
B.染色体畸变
C.DNA完整性改变
D.细胞坏死
E.细胞恶性转化