partners cannot manage without each other. This is so in the corals (珊瑚) of the sea. In their skins they have tiny plants which act as "dustmen", taking some of the waste products from the coral and giving in return oxygen which the animal needs to breathe. If the plants are killed, or are even prevented from receiving light so that they cannot live normally, the corals will die.

21. Some birds like to sit on a sheep because _______.

A. they can eat its parasites B. they enjoy travelling with the sheep

C. they depend on the sheep for existence D. they find the position most comfortable

22. The underlined word "they" in the last sentence of the first paragraph refers to _______.

A. birds and parasites B. birds and sheep

C. parasites and sheep D. sheep, birds and parasites

23. We learn from the text that corals depend on plants for _______.

A. comfort B. light C. food D. oxygen

24. What does the second paragraph mainly discuss?

A. Some animals and plants depend on each other for existence.

B. Some animals and plants develop their relationship easily.

C. Some plants depend on each other for food.

D. Some animals live better together.

　　I have now been in Spain for 81 days. Last week, I went to watch the Christmas lights being switched on in the city centre of Santander, marking the countdown to Christmas. I will be spending my Christmas in the UK, something that I usually hate, since I don't like spending money. However, having been in Spain for nearly three months, I have realized four things that I really miss about Christmas in the UK.

Advent calendars (降临节日历)

Although I have seen them on sale in shops here in Spain, not one of my students or friends here has bought one. Meanwhile, my friends in the UK have all been posting photos of their Advent calendars on Facebook and Instagram, celebrating the start of December. Because who doesn't want the excuse to eat chocolate every day for a