痢疾杆菌的主要致病因素是（）。
A.吞入细菌数量
B.外毒素
C.神经毒素
D.侵袭力和内毒素
E.肠毒素
参考答案：D
完成下图的光路图
Speaker A: Do you mind if I sit down hereSpeaker B: ______
A．Of course. Do sit down, please.
B．Not at all. There's plenty of room.
C．Never mind. I'd like to have some companion.
D．Sure, I'd like to have somebody to talk to.
