-They are quiet, aren't they?

- Yes. They are accustomed ________ at meals.[ ]

A．to talk  

B．to not talk

C．to talking  

D．to not talking

细菌内毒素的特征是（）。

A．只有革兰阴性细菌产生

B．少数革兰阳性细菌产生

C．细菌在生活状态下释放

D．抗原性强

E．不耐热