在蒸馏水中加入一定量的苯，培养洋葱根尖一段时间，观察有丝分裂中期染色体的形态和数目。下列相关叙述合理的是（）

题干

A：实验目的可能是研究苯对染色体结构和数目的影响，但还需设计对照组

B：换用高倍镜观察染色体时，调节光圈、反光镜就可使视野清晰

C：预计根尖细胞中染色体的数目都是4N，但形态都不同

D：依据的原理之一是“致癌物质苯会使细胞发生基因突变”

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