题目： Prairie Diversity 1 Prairies—the grasslands that once covered immense areas of central North America—were created and maintained by factors that favored the growth of g...

题目： The Unity of China Around 1400,Islamic societies were the most dynamic and expansionist political entities on the Eurasian continent.But it was China whose wealth and power...

题目: Calcium and Eggshells   Creating an egg requires various nutrients, among which the calcium for the shell is the most difficult to obtain. This is partly because most bi...

题目：   Peruvian farmers have used guano (dried waste of seabirds) as a plant fertilizer since ancient times, but guano was not exported to Europe until 1840. Guano has mu...

题目：   Planetary scientists have struggled to understand how three rocky planets could have turned out so differently. Why is the atmosphere on Venus 100 times as dense as ...

The Birth of the Solar System The Birth of the Solar System     题目：   Scientists believe that the sun and the planets in our solar system formed from a vast clo...

题目： How Humans Became Food Producers   Present evidence indicates that the earliest plant domestication (the process of adapting wild plants for human use)took place gradu...

题目： Hot Springs and Geysers     Water held in the cracks in rocks and spaces in soil is called groundwater. A hot spring forms when hot groundwater rises through a c...

题目： The Origin of Writing Who invented the first writing? Like many questions of its type in archaeology, this one is not easily answered.Some might argue that the paintings ...

  题目： Endangered Coral Reefs     Reef-building corals are made by polyps (tiny sea animals) that build on the skeletal remains of previous generations provid...