能抑制胶原酶活性的是（） A.甲硝唑 B.四环素 C.氯己定含漱剂 D.1%过氧化氢

如图所示，一质量为m．长为L的金属杆ab，以一定的初速度v从一光滑 平行金属轨道的底端向上滑行，轨道平面与水平面成θ角，轨道平面处于磁 感应强度为B、方向垂直轨道平面向上的磁场中．两导轨上端用一沮值为R的电阻相连，轨道与金属扞ab的电阻均不计，金属杆向上滑行到某一高度 后又返回到底端．则金属杆（　　）

A．在上滑过程中安培力的冲量大小为mv₀

B．在上滑过程中克服安培力做的功大于下滑过程中克服安培力做的功

C．在上滑过程中电阻R上产生的焦耳热等于减少的动能

D．在上滑过程中流过电阻R的电荷量大于下滑过程中流过电阻R的电荷量

Because of the development of agriculture, the area of land affected by sand has gradually increased. During the 1950s and 1960s the land affected by sand expanded on average into 1,600 square kilometers per year. By the 1990s, the area had reached 2,500 square kilometers per year.  This kind of situation is particularly serious in many regions upwind(逆风的) from Beijing. Fengning City in Hebei Province and Duolun City in Inner Mongolia lie to the north of Beijing, and in the last 50 years the population of these two cities has grown by 2 to 3 times. In order to obtain more resources, the inhabitants have been busy with  heavy farming, tree-cutting and keeping animals. The result is that the soil which has sustained damage, and the sand layer below the earth is now exposed on the ground and the ground is rapidly being controlled by the spread of sand.

China has already noticed this problem, and has take measures to save the land. Nowadays, some land used for farming has returned to forestland and grassland. The Chinese government announced that it would invest more than RMB 600,000,000 to control Beijing’s sandstorms in the coming years. Only by decreasing the damage caused by man and restoring the natural environment will it be possible to well control the sources of the sand and dust. We hope in the not-distant future, we will solve the problem successfully and effectively.  

63. Compared with 50s and 60s, the land affected by sand increased by _____ square kilometers by the 1990s.

A. 1,600        B. 2,500         C. 900          D. 1,100         

64. Which of the following is NOT the cause of the damaged land?                      

A.People use the resources too much.

B.People keep animals which need more grain.

C.People prepare too much land to grow crops.

D.People cut a lot of forests and raise animals.

65. The major measure that Chinese government has taken is _____.                                                          

A.controlling the growth of the population     

B. making better use of natural resources

C. killing fewer animals and cutting fewer trees   

D. returning some land to forestland and grassland

66.According to the passage, it can be inferred that _____ was the major factor of the environmental damage.

A. man       B. animal      C. nature    D. desert 

我们使用PowerPoint进行课件制作时，默认情况下，插入一张新幻灯片操作的快捷键是（）。

在通常情况下，从追求寿命周期成本最低的立场出发，应该( )。

A．确定寿命周期成本的各要素，把各要素的成本降低到普通水平

B．从寿命周期成本和系统效率的关系这个角度进行研究

C．将购置费和维持费两者进行权衡，以便确定研究的侧重点，从而使总费用更为经济

D．由于寿命周期成本是在长期内发生的，对费用发生的时间顺序必须加以掌握

E．在寿命周期成本中不必考虑资金的时间价值

在极重、特重和重交通高速、一级公路的桥涵上及各级公路翻修困难、要求使用年限较长的钢筋混凝土桥面铺装中，可采用（）

A.聚丙烯腈纤维混凝土

B.钢纤维增强混凝土

C.纤维钢筋混凝土

D.聚合物水泥混凝土