On Halloween, Children in costumes ask for candies ____________ shouting “ Trick or Treat”.

A．from door to door

B．from one door to door

C．from one door to one door

D．drom door to another

DNA变性后，将溶液立即放在冰上，这是为了（）。

A.让DNA变硬

B.让DNA沉淀

C.使DNA分解破坏

D.防止DNA降解

E.保持DNA的单链状态