How to avoid multiple plasmid bands in the gel?

为什么质粒DNA中有多条带？

A) 可能是DNA提取过程中混合太剧烈; B) 加入PB20处理时间太长了。

A) After addition of PB20, mix too vigorously; B) wait too long after adding PB20; C) Cells were vigorous growth.