Digitalis is a drug which is prepared from the seeds and leafs of a plant with the same name and is used as a cardiac stimulant.

有关质粒的叙述，不正确的为（）

A.质粒是细菌核质以外的遗传物质

B.质粒是细菌必备结构

C.质粒不是细菌必备结构

D.质粒是双股环状DNA

E.质粒可独立存在于细菌体内