近似运算是数学科学的重要分支之一，很多数值都是无理数，我们无法进行下一步运算，因

一个登山队，上山时每小时行7千米，下山每小时行9千米，往返用8小时，求这个登山队上、下山的平均速度．

Around 200 million years ago, the earth was 18 degrees warmer than it is now.  That might be a bit hot for humans, but it was just right for the giant dinosaurs that wandered about the earth during the Mesozoic (中生代的) era. A new study found that planteating dinosaurs may have contributed to the warming of the earth­by releasing significant amounts of methane (甲烷)．

The study, published in science journal Current Biology, focused on sauropods, long­necked herbivores (食草动物) that munched (用力咀嚼) on the top of trees. They  were the largest of the dinosaurs; food broke down in their  stomachs for long periods of time.  The researchers estimate that a sauropoda released 2,675 liters of methane per  day—adding up to a large amount of the greenhouse gas  being pumped from dinosaur's gut into the atmosphere.

Plant eaters naturally release methane as part of their   digestive process. The larger the animal, the more methane  it produces. Methane, a greenhouse gas, traps heat and remains in the atmosphere for approximately 9­15 years,  which warms the atmosphere.

Today,  some  animals,  such  as  cattle,  goats  and  sheep, produce large amounts of methane as a by­product  to their food digestion. Methane is also released from human—influenced sources such as landfill, agricultural activities, coal mining and other industrial practices—which all contribute to today's climate change, according to the  Environmental Protection Agency.

The researchers estimate that earth's sauropods would  have produced about 520 million tons of methane per  year—similar to the total amount of methane produced today by natural and man­made sources.  However, the researchers warn that their numbers are estimates based on  multiple assumptions about the digestive systems and populations of dinosaurs.

The author of the study David Wilkinson says the  emission of methane from dinosaurs would have been just  one of the causes of warm climate during the Mesozoic era.  Other  causes  include  gases  produced  from  volcanoes,  swamps, shallow seas and so on. Still, Wilkinson says，“  The amount of methane from dinosaurs is large enough to  have a measurable effect.”

小题1:Scientists think dinosaurs may have led to the global     warming because ________.

A．they ate everything they found

B．they usually had a good appetite

C．they released a lot of methane

D．there were too many dinosaurs on earth小题2:What do we know about dinosaurs from Paragraph 2?

A．Sauropods fed on plants.

B．Sauropods were of medium size.

C．Dinosaurs used to live on trees.

D．Dinosaurs digested food quickly.小题3:Which  of  the  following  activities  doesn't  produce     methane?

A．Coal is dug out by machine.

B．Trees and flowers are planted.

C．Goats digest grass under the tree.

D．Food is being processed in the factory.小题4:How does the author make his writing reliable?

A．By listing some facts.

B．By making predictions.

C．By citing figures.

D．By using mind pictures.小题5:David Wilkinson seems to believe that ________.

A．methane had little effects on the earth

B．many other factors contributed to warm climate

C．other causes for warm climate were worth considering

D．dinosaurs were mainly to blame for global warming

下列选项中关于社会工作理论在社会实务中的预测作用，说法正确的是()。

A.对求助者对服务的满意度作出预测

B.对求助者可能发生的变化作出预测

C.对社会工作者提供服务对求助者可能产生的效用作出预测

D.对谁将成为服务对象作出预测

E.对影响求助者的各种因素的作用作出预测

补体系统的概念中，“在病毒与相应抗体形成的复合物中加入补体，则明显增强抗体对病毒的中和作用，阻止病毒对宿主细胞的吸附和穿入”属于（）

A.补体系统

B.补体受体

C.调理作用

D.补体的激活

E.中和及溶解病毒作用

特写符号