ppm是什么意思？它与百分率之间如何换算？

Mr Smith works in a factory in the middle of the city. The factory is not far from his home. It is about 800 meters away. So Mr Smith doesn’t go to work by bus. He usually goes there by bike, sometimes he walks to work. It takes(花费) him 10 minutes to get there by bike or 20 minutes on foot. He usually leaves home at ten minutes to eight. Today something is wrong with his bike. He wants to walk there. His work starts at half past eight in the morning and finishes at a quarter(一刻钟) to five in the afternoon.

根据短文内容，选择正确答案。

小题1: What does Mr Smith do?

A．He is a worker.

B．He is a teacher.

C．He is a doctor.

D．He is a farmer.小题2: Why doesn’t Mr Smith go to work by bus?

A．Because there is no bus.

B．Because his factory is not far from his home.

C．Because he likes riding a bike.

D．Because he doesn’t like taking a bus.小题3: How long does it take Mr Smith to walk to work?

A．Twenty minutes.

B．Forty minutes.

C．Ten minutes.

D．Half an hour.小题4: What time does Mr Smith usually leave home?

A．8:10.

B．7:50.

C．7:10.

D．8:50.小题5: Mr Smith usually goes home from work at ______ in the afternoon.

A．4:15.

B．4:45.

C．4:35.

D．4:05.

中国特色社会主义理论体系，就是包括___________________、“三个代表”重要思想以及_______________________等重大战略思想在内的科学理论体系。

{a_n}为等差数列，前n项和为S_n，S₁₀=5，S₂₀=10，则S₃₀=（    ）。

继发性三叉神经痛与原发性三叉神经痛的最重要鉴别点（）

A.有扳机点

B.有其他神经系统体征

C.常有牙疾患

D.疼痛的程度

E.卡马西平的治疗效果

蜗杆传动装配时，蜗杆、蜗轮的正确接触点应在（）位置。

A、蜗轮偏左

B、蜗轮偏右

C、蜗轮中部稍偏蜗杆旋入方向

D、蜗轮中部稍偏蜗杆旋出方向