How would you like to live in a land where the sun shines day and night? There are place in the polar regions where, for North, the sun never sets! This is known as the land of the Midnight Sun. This period of sunlight can last up to six months.

This happens because the Earth tilts(倾斜). When the North Pole is tilted towards the sun, it has constant (不断) daylight. As the same time, the South Pole is tilted away from the sun. It is in darkness.

Six months later, the South Pole tilts to the sun and has constant daylight. Then the North Pole is tilts away. It has darkness.

小题1: Where can you see the Midnight Sun?

小题2:How long can the period of sunlight last?

小题3:Why can we see the Midnight Sun?

小题4:When there is Midnight Sun in the North Pole?

小题5:What happens in the South Pole?

N-乙酰葡糖胺与N-乙酰胞壁酸通过什么化学键联结形成细胞壁聚糖骨架？（）

A.二硫键

B.β-1，4糖苷键

C.氢键

D.α-1，4糖苷键

E.肽键