实验动物新生期摘除胸腺( ) A．脾脏红髓区T细胞缺乏 B．淋巴结副皮质区T细胞缺乏

负有代扣代缴义务的单位、个人是( )。

A．(A) 实际负税人

B．(B) 扣缴义务人

C．(C) 纳税义务人

D．(D) 税务机关

如图所示，一固定在竖直平面内的光滑半圆形轨道ABC，半径为R=0.5m，轨道在C处与粗糙的水平面相切，在D处有一质量m=1kg的小物体压缩着弹簧，在弹力的作用下以一定的初速度水平向左运动，物体与水平面间的动摩擦因数为0.25，物体通过C点后进入圆轨道运动，恰好能通过半圆轨道的最高点A，最后又落回水平面上的D点（g=10m/s²,不计空气阻力），

求：(1)物体到C点时的速度 ;

（2）弹簧对物体做的功。

传动皮带为什么要适当地张紧？

肌肉的物理特性包括伸展性、（）和（）。

Believe it or not，optical illusion can cut highway crashes.

Japan is a case in point．It has reduced automobile crashes on some roads by Rearly 75 percent using a simple optical illusion．Butst．ripes,called chevrons(人字形)，painted on ther rads make drivers think that they are driving faster than they really are，and thus drivers slow down.

Now the American Automobile Association Foundation for Traffic Safety in Washington D．C. is planning t。repeat Japan’s success．Starting next year，the foundati。n will paint cheyrons and。ther pattems of stripes on selected roads around the country to test how well the patterns reduce highway erashes．

Excessive(too great)speed plays a major role in as much as one fifth of all fatal traffic accidents， according to the foundation．To help reduce those accidents，the foundation will conduct its tests in areas where speed-related hazards(danger)are the greatest curves，exit slopes，traffic circles，and bridges．

Some studies suggest that straight，horizontal bars painted across roads can initially cut the average speed of drivers in half．However,traffic。ften returns to full speed within months as driVers become used to seeing the painted bars．

Chevrons，scientists say，not only give drivers the impression that they are driving faster than they reallv are but als0 make a lane appear to be narrower．The result is a longer lasting reduction in highway speed and the number of traffic accidents．

The American Automobile Association Foundation for Traffic Safety plans to（）

A. try out the Japanese method in certain areas

B. change the road signs across the country

C. replace straight, horizontal bars with chevrons

D. repeat the Japanese road patterns