对高危人群开展干预活动，有针对性地选择有效干预方法，提高抵挡毒品诱惑力的预防是（）。

阅读下面短文。在短文的空格内填入适当的词，使其内容通顺。(每空格限填一词)

There was once a rich man who loved his little boy very

much. He wanted to try his best to please (使……高兴) him.

So he gave him a horse to ride, beautiful rooms to live in,

pictures, books, toys and everything that money  小题1: buy.

But for all this, the little boy was still  小题2: . He wore a frown ( 皱眉 ) wherever he went, and always wished for something he did not have. The man didn' t know how to make his son happy.

One day, a magician (魔术师) came to the man and said to him, "I can make your son happy and turn his frowns into smiles, but you must pay me a great price for 小题3: him the secret. "

"All right," said the man, "No matter what you ask for, I’ll give you." The magician took the boy into a secret room. He wrote something on a piece of paper, and then gave it to the   小题4:There on the paper came the words, "Do one kind thing for someone every day. " The boy  小题5: the advice and became one of the happiest boys.

Only those who stop thinking about their own happiness can be truly happy.

治疗钩端螺旋体病最佳的药物是（）。

A.链霉素

B.青霉素G

C.红霉素

D.氯霉素

E.四环素

如图甲所示，长方形金属框abcd（下面简称方框）。各边长度为ac=bd=1/2、ab=cd=l，方框外侧套着一个内侧壁长分别为l/2及t的u形金属框架MNPQ（下面简称U形框），U形框与方框之间接触良好且无摩擦。两个金属框的质量均为m，PQ边、ab边和cd边的电阻均为r，其余各边电阻可忽略不计，将两个金属框放在静止在水平地面上的矩形粗糙平面上，将平面的一端缓慢抬起，直到这两个金属框都恰能在此平面上匀速下滑，这时平面与地面的夹角为θ，此时将平面固定构成一个倾角为θ的斜面，已知两框与斜面间的最大静摩擦力近似等于滑动摩擦力，在斜面上有两条与斜面底边垂直的、电阻可忽略不计，且足够长的光滑金属轨道，两轨道间的宽度略大于l，使两轨道能与U形框保持良好接触，在轨道上端接有电压传感器并与计算机相连，如图乙所示，在轨道所在空间存在垂直于轨道平面向下、磁感应强度大小为B的匀强磁场。

（1）若将方框固定不动，用与斜面平行，且垂直PQ边向下的力拉动U形框，使它匀速向下运动，在U形框与方框分离之前，计算机上显示的电压为恒定电压U_o，求U形框向下运动的速度；

（2）若方框开始时静止但不固定在斜面上，给U形框垂直PQ边沿斜面向下的初速度v₀，如果U形框与方框能不分离而一起运动，求在这一过程中电流通过方框产生的焦耳热；

（3）若方框开始时静止但不固定在斜面上，给U形框垂直PQ边沿斜面向下的初速度3v₀，U形框与方框将会分离，求在二者分离之前U形框速度减小到2v₀时，方框的加速度。

注：两个电动势均为E、内阻均为r的直流电源，若并联在一起，可等效为电动势仍为E，内电阻为r/2的电源；若串联在一起，可等效为电动势为2E，内电阻为2r 的电源。

自主选择权

城市基础设施的特性？