安装螺栓时为什么应选择螺母材质比螺栓材质低一级？

阅读理解。

    A plant makes its own food in its leaves. Water comes to the leaves through the roots ( ). Air gets into

the leaves through very small holes. The green coloring ( ) in the leaves uses the water and air to make the

food for the plant. It also needs sunshine because a plant can make food only when the sun is shining. Animals

and people could not live without green plants. They both eat plants. People and some animals also eat the meat

of some animals and these animals eat plants.

1. A plant gets food from           . [ ]

A. the sun

B. the air

C. its roots

D. its leaves

2. What things can make the food for a plant? [ ]

A. Sunshine, water, air and the roots.

B. Water, air, meat and green coloring.

C. Air, sunshine, water and green coloring.

D. Water, sunshine, animals and green coloring.

3. A. plant has very small holes in its leaves           . [ ]

A. for water to come in

B. for air to come in

C. for sunshine to come in

D. for green coloring to come in

4. A plant can make its food when it is            . [ ]

A. sunny

B. rainy

C. cloudy

D. windy

5. From the text we know            . [ ]

A. people can't get food without animals.

B. people can't get plants without animals.

C. people can't get animals with out meat.

D. people can't get food without plants.

AMI患者CK变化不正确的是（）

A.发病3～8小时即明显增高

B.峰值在10～36小时

C.3～4天恢复正常

D.发病8小时内CK不增高可排除AMI

E.病程中CK再次升高提示再梗死

化学自净作用是（）。

A．环境污染物的氧化、还原、分解等

B．环境污染物通过微生物的降解

C．环境污染物自动分解

D．环境污染物通过稀释、扩散、挥发等

E．环境污染物自动降解

如图所示，质量为70kg的工人站在岸边通过一滑轮组打捞一块沉没在水池底部的石材，该滑轮组中动滑轮质量为5kg．当工人用120N的力拉滑轮组的绳端时，石材仍沉在水底不动．工人继续增大拉力将石材拉起，在整个提升过程中，石材始终以0.2m/s的速度匀速上升．在石材还没有露出水面之前滑轮组的机械效率为η₁，当石材完全露出水面之后滑轮组的机械效率为η₂，且η₁：η₂=63：65．绳重及滑轮的摩擦均可忽略不计，石材的密度

ρ_石=2.5×10³kg/m³，取g=10N/kg，求：

（1）当人用120N的力拉绳端时，岸边地面对人的支持力为多大；

（2）在石材脱离水池底部至完全露出水面的过程中，地面对人的支持力的最大值与最小值之比；

（3）当石材露出水面之前，人拉绳子的功率；

（4）此人用这个滑轮组提升重物的最大机械效率．

集部