New words：

shortage ['ʃɔ:tidʒ] n. 短缺；不足；缺少

Sample Sentences：

Rescuers are facing a shortage of supplies.
There is a shortage of fresh water on the island.
There was a shortage of oxygen at the top of the mountain.

New words：

severe [si'viə] adj. 极为恶劣的；十分严重的；严厉的

Sample Sentences：

His injuries are severe.
The general had a severe manner.
My uncle became severe when I was late.
The bad harvest led to severe food shortage.
She received severe head injuries in the accident.

shortage ['ʃɔ:tidʒ] n. 短缺；不足；缺少

New words：

pollution [pə'lu:ʃən] n. 污染；污染物

Sample Sentences：

We have freed the lake from pollution.
The new law will reduce pollution of the rivers.
We are sad about the pollution of our beaches with oil.
Many beautiful fish are fast disappearing because of the severe pollution.

severe [si'viə] adj. 极为恶劣的；十分严重的；严厉的

New words：

pollute [pə'lu:t] v. 污染；弄脏

Sample Sentences：

Garbage pollutes our rivers and lakes.
I feel it is our duty not to pollute our environment.
The river has been polluted by waste products from the factory.
Many things pollute water, such as tires, trash, and plastic bags.

New words：

default [di'fɔ:lt]
n.违约；拖欠；缺席；缺陷；默认；系统设定值，预置值
v.违约；拖欠；缺乏；不履行

Sample Sentences：

The team lost a game by default.
Some people could decide to default.
If they stop paying, that's called a default.
You can type a different name, or use the default name.
But not every Android phone uses Google's maps app by default.
If default happens and they are blamed, they will pay a terrible price.