New words：
optional [ˈɒpʃənl] adj. 可选择的；选修的；随意的
Sample Sentences：
The goods are optional, but only one.
Some courses in college are required while others are optional.
Is English an optional lesson, or does everyone have to learn it?
course /kɔːrs/ n. 课程
New words：
option [ˈɒpʃn] n.选项；选择；选择权；可选择的事物
Sample Sentences：
I had no option but to ask him to leave.
We need to consider all available options before making a decision.
The government has two options, to reduce spending or to increase taxes
New words：
denude [dɪˈnjuːd] v.剥光；使裸露；使光秃
Sample Sentences：
She goes up the steps to denude the statue.
Most trees are denuded of their leaves in winter.
Many hillsides had been denuded of trees.
New words：
mine [maɪn]
n.矿；矿井；地雷；水雷
v.采矿；布雷；埋雷于；用雷炸毁（车辆）
mining [ˈmaɪnɪŋ] n. 采矿；采矿业
Sample Sentences：
Mining is one of the country's main industries.
The engine is used for pumping water out of the mine. Mining would pollute the lake and denude the forest.
The mines had first been worked in 1849, when gold was discovered in California.
denude [dɪˈnjuːd] v.剥光；使裸露；使光秃
New words：
industrial [ɪnˈdʌstriəl] adj.工业的；产业的；
Sample Sentences：
This chemical has a wide range of industrial uses.
Tianjin is an old industrial city, is an important industrial base.