综合治理长江的首要任务是[ ] A. 防洪 B. 治理水污染 C. 疏通河道

如图所示，一根长为l的细刚性轻杆的两端分别连结小球a和b，它们的质量分别为m_a和 m_b。杆可绕距a球为处的水平定轴O在竖直平面内转动。初始时杆处于竖直位置。小球b几乎接触桌面。在杆的右边水平桌面上，紧挨着细杆放着一个质量为m的立方体匀质物块，图中ABCD为过立方体中心且与细杆共面的截面。现用一水平恒力F作用于a球上，使之绕轴逆时针转动，求当a转过角时小球b速度的大小。设在此过程中立方体物块没有发生转动，且小球b与立方体物块始终接触没有分离。不计一切摩擦。

（）工件的磨削表面是由砂轮上大量磨粒刻划出无数极细的刻痕形成的，工件单位面积上通过的砂粒数越多，则刻痕越多，刻痕的等高性越好，表面粗糙度值越小。

李红同学设计了四个历史小论文题目，其中出错的是（    ）

A、瓦特与蒸汽机                       B英国与三角贸易

C、章西女王与拉美独立革命             D、亚历山大与1861年改革

When Dave was eighteen, he bought a secondhand car for 200 so that he could travel to and from work more (1) than by bus. It worked quite well for a few years, but then it got so old, and it was costing him (2) much in repairs that he decided that he had better (3) it.

He asked among his friends to see if anyone was particularly (4) to buy a cheap car, but they all knew that it was falling to pieces, so (5) of them had any desire to buy it. Dave’s friend Sam saw that he was (6) when they met one evening, and said, "What’s (7) , Dave"

Dave told him, and Sam answered, "Well, what about advertising it in the paper You may (8) more for it that way than the cost of the advertisement!" Thinking that Sam’s (9) was sensible, he put an advertisement in an evening paper, which read "For sale: small car, (10) very little petrol, only two owners. Bargain at 50. " For two days after the advertisement first appeared, there was no (11) . But then on Saturday evening he had an enquiry. A man rang up and said he would like to (12) him about the car. "All right, " Dave said, feeling happy. He asked the man whether ten o’clock the next morning would be (13) or not. "Fine, " the man said, "and I’ll (14) my wife. We intend to go for a ride in it to (15) it. "

The next morning, at a quarter to ten, Dave parked the car in the square outside his front door, (16) to wait there for the people who had (17) his advertisement. Even Dave had to (18) that the car really looked like a wreck. Then, soon after he had got the car as clean (19) it could be, a police car stopped just behind him and a policeman got out. He looked at Dave’s car and then said, "Have you reported this (20) to us yet, sir\

3().

A.keep 

B.repair 

C.sell 

D.throw

水资源在城市发展中的重要作用