单向阀能否作背压阀使用？为什么？

虽然许多没有大学学历的人也能成为世界著名的企业家，比如微软公司的创始人之一比尔•盖茨，就没有正式得到大学毕业文凭，但大多数优秀的管理人才还是接受过大学教育特别是MBA教育。虽然得到MBA学位并不意味着成功，但还是可以说MBA教育是培养现代企业管理人才的摇篮。 以下论断除了哪项外，都可能是以上题干的文中之义

A．有些人在大学里是学习哲学的，搞起经营管理来却不比学MBA的差。

B．对于有些天才人物，不经历MBA教育阶段也可以学到MBA教育传授的知识和才能。

C．由于MBA教育离实际的管理还有一定距离，得到MBA学位的人还需要在实践中不断积累管理经验。

D．得到MBA学位的学生毕业后，大多数人成为优秀的管理人才，有些人成为世界知名企业高级主管。

阅读理解。

      Five million people visit Grand Canyon in the US every year. For the purpose of helping protect Grand

Canyon for your fellow visitors and future generations, please follow the guidelines below.

Camping 

To protect the park, camping is allowed only within permitted campgrounds. Permits (许可证) are required

for overnight camping at the North Rim. Advance booking can be received by mail. Please write: Information

Center, P.O. Box 129, Grand Canyon, AZ 86023

Fires

Because of the fire danger, campfires are not allowed except at Mather and Desert View campgrounds.

Collection of firewood is not allowed either.

Hiking

Please stay on permitted paths. Otherwise you may destroy desert plants. Pack out what you pack in, so you

leave no signs of your visit. It is important to keep in mind that you are in a national park where wildlife exists.

Weather

The weather at Grand Canyon can change very quickly. With so much rock, lightning causes a particular

danger during sudden summer storms. These storms also frequently bring floods inside valleys, a danger to

hikers. Watch the skies and check daily weather reports.

Wildlife

Do not feed park wildlife. There have been a few cases at Grand Canyon National Park where deer were

purposely shot because they ate plastic bags that left them sick and weak. Hungry deer can be dangerous and

they have kicked and bitten visitors at Grand Canyon. Some other animals will also beg and bite. For your own

safety and the well-being of the animals, please do not feed wildlife, no matter how gentle they may appear.

1. What can you do first if you want to go camping? [ ]

A. Know the permitted

B. Book campgrounds in advance

C. Make sure not to make a fire

D. Stop at Mather and Desert View

2. What do the underlined words "pack out what you pack in" in Paragraph 4 probably mean? [ ]

A. keep every out of campgrounds

B. take away everything you bring in

C. carry all the necessary food

D. look after your personal belongings

3. Why were some deer killed on purpose at Grand Canyon? [ ]

A. They ate wrong things and became very ill.

B. They were a danger to other gentle animals.

C. They begged food from visitors.

D. They kicked and bit visitors.

4. What is the main purpose of the text? [ ]

A. To provide travel information.

B. To report some recent news.

C. To teach tourists hiking skills.

D. To introduce the wild life.

当额定起重量不大于30t用于吊运熔融金属的起重机，可以不设置超速保护装置。

工程量以m³为计量单位的是（）

A.平整场地

B.挖基础土方

C.挖沟槽土方

D.土方回填

袋收尘在开机前要确认哪些（）

A、袋收尘内无人

B、无工具及杂物

C、高压气体压力达到要求

D、人孔门关闭