---Will you please have dinner with me tonight?

---         . I’ll have something more important to do.

A．Sorry, I can’t

B．Yes, I’d like to.

C．Let’s go

D．No, I don’t think so.

DNA在水溶解中热变性之后，如果将溶液迅速冷却，则DNA保持______状态；若使溶液缓慢冷却，则DNA重新形成______。