在观察DNA和RNA在细胞中的分布实验中，使用盐酸的目的是

①改变细胞膜的流动性 ②改变细胞膜的通透性 ③改变细胞膜的活性 ④DNA与蛋白质分离 ⑤RNA与蛋白质分离 ⑥有利于RNA与染色剂结合 ⑦有利于蛋白质与染色剂结合 ⑧有利于DNA与染色剂结合[ ]

A．①④⑥

B．②④⑧

C．③⑤⑦

D．②⑤⑥

根据对话语境和所给首字母提示，填写正确的单词完成对话

A: Good morning.I'd like to see Mr Brown.

B: Do you have an a  1   ?

A: No,I'm a  2   not.I've just arrived in the town today.

B: Mr Brown is quite busy.I'm afraid he won't be able to see you today.

A: Do you think I can see him t  3   ?

B: Let me see. He's f  4   at 2:30 tomorrow afternoon. Can you come here on t  5   ?

A: Yes,I'll be here at 2:30. Thank you.

B: You are w  6  . Goodbye.