简述不当得利的概念和成立条件。

阅读理解。

    An American group developed the rooftop gardening in the early 1980s. The group is known as ECHO.

This idea was to help people living in cities who did not have land to grow vegetables. And now it is

becoming more and more popular in many other countries.

    To build a rooftop garden, four things are necessary. You need a roof strong enough and four pieces of

wood to build a box, and also you need grass cuttings in which to plant your garden and a thin piece of plastic

on which to put the cut grass.

    Begin by testing your roof to make sure it is strong enough. Cut and collect some grass. Cover the roof

with a thin piece of plastic. Then you build a box from the four pieces of wood. The box has no top or

bottom. It should be about 8cm deep. It can be as long and as wide as needed. Put the box on the plastic.

Then fill in with the cut grass. Water the grass and push it down by walking on it. After about three weeks,

the rooftop box is ready for planting.

    Put the seeds (种子) directly into the wet grass. It is important that the grass is kept wet until the plants

begin to grow. When the plants are growing, you will need to water them every day unless it rains. And birds

must be prevented from eating the seeds or new plants. They will also need some liquid fertilizer (液体化肥).

You can make your own liquid fertilizer if you can get waste (粪便) from chickens. Put the chicken waste in

a cloth bag. Then put the bag in a big container (容器) of water. After about one week, the water becomes a

good liquid fertilizer.

    The rooftop garden is a good place to grow peas, tomatoes and onions. If the box is deep enough, potatoes

and carrots will also grow.

1. Which of the following is NOT the thing you need to build a rooftop garden?[ ]

A. A thin piece of plastic

B. A big container and a bag

C. Grass cutting to plant your garden in

D. Four pieces of wood and a strong roof

2. When the plants are growing,            . [ ]

A. you don't need to water them

B. you need to water them every day

C. you don't need to water them if it rains

D. it's not necessary to water them at all

3. After planting the seeds, grass must be kept            . [ ]

A. wet

B. dry

C. warm

D. cool

4. The passage is mainly about the way to            . [ ]

A. make liquid fertilizer

B. make a rooftop box

C. cover the roof with grass

D. grow vegetables without land

已知数列{a_n}满足条件a₁=0，a_n+1=a_n+2n（n∈N*），那么a₂₀₁₀=（    ）。

患儿，8岁。诊断为猩红热。现身热渐退，咽喉糜烂，疼痛减轻，皮疹渐消，唇口干燥，食欲不振，舌红少津，脉细。其证型是（）

A.邪侵肺卫

B.疹后伤阴

C.毒在气营

D.阴虚火旺

E.肺脾气虚

骨软化症的特点不包括（）。

A.患者多于青春期发病，与维生素D缺乏有关

B.主要表现为骨痛、肌无力，还可出现骨骼变形

C.X线检查有助于本病的诊断

D.以骨基质矿化障碍为特点的一种疾病

E.血清碱性磷酸酶可轻、中度升高

雏鸡缺乏下列哪种物质时患“多发性神经炎”。（）

A、维生索PP

B、维生素B1

C、维生素B2

D、维生素B6