全球卫星定位系统（缩写为GPS）就是卫星通信的实际应用，导航仪接收到的信息是通过

以下是某历史小泽同学为云冈石窟涉及的解说词，其中正确的是 [ ]

A．开凿于两汉，宣传的是佛教

B．开凿于东汉，宣传的是道教

C．开凿于北朝，宣传的是佛教

D．开凿于唐朝，宣传的是道教

世界贸易组织的前身是( )。

A．联合国经社理事会

B．海关合作理事会

C．关贸总协定

D．国际商会

After graduation she reached a point in her career ___   she needed to decide what to do.[    ]

A. that

B. what    

C. which

D. where

As viewed from space, the Earth’’s distinguishing characteristics are its blue waters and white clouds. Enveloped by an ocean of air consisting of 78% nitrogen and 21% oxygen, the planet is the only one in our solar system known to harbor life. Circling the Sun at an average distance of 149 million km (93 million miles), the Earth is the third planet from the Sun and the fifth largest planet in the solar system. Its rapid spin and molten nickel-iron core give rise to an extensive magnetic field which, coupled with the atmosphere, shields us from nearly all of the harmful radiation coming from the Sun and other stars. Most meteors burn up in the Earth’’s atmosphere before they can strike the surface. The planet’’s active geological processes have left no evidence of the ancient pelting it almost certainly received soon after it was formed. The Earth has a single natural satellite — the Moon.

What two factors help protect the Earth from radiation

A．Magnetic field and atmosphere.

B．Rapid spin and molten iron-nickel core.

C．The Sun and the Moon.

D．Blue waters and white clouds.

生化耗氧量是什么？