题目

冰面对滑冰运动员的最大摩擦力为其重力的k倍，在水平冰面上沿半径为R的圆周滑行的运动员，若仅依靠摩擦力来提供向心力而不冲出圆形滑道，其运动的速度应满足：(    )A.    B．    C.    D． 答案：B10．Mountain lions live in mountainous areas of the western United States．Lately，however，people have reported seeing them in quiet neighborhoods all across the country!Usually considered shy and seldom seen，these lions are becoming more noticeable．In addition，the lions appear to be moving east．They were even reported in the suburbs of Boston，Philadelphia，and Chicago．It seems certain that these"kings of the mountains"are moving．Why？Scientists are not sure．Some think that as the mountain lion population increases in the west，younger wild cats are leaving the area．These cats are searching for new homes and new places for food．"The young males are looking for new homes，"says Lynn Sadler，who is the director of the Mountain Lion Foundation in Sacramento，Calif．"People tend to create too many barriers，such as buildings，fences，roads，and noise．As the mountain lions seek new places and food and water sources，these barriers get in their way．"The sightings have made people increasingly watchful．Sometimes the"mountain lions"that people report seeing turn out to be large dogs or even house cats．But at least a few of the meetings have been real．The building of new roads，t he growing of suburbs and the growing popularity of outdoo r activities are all increasing the chances that people in various parts of the country may meet up with a mountain lion．Because many people are concerned about the reappearance of the American lion，an expert outlined some ways they can avoid attracting the wild cats：Never leave pets or pet food outside； properly care for farm animals； and plan public land use to enable the lions to move safely from one place to another．"To be fair，"says the expert，"mountain lions are trying to regain their homes．They were here first．Now we need to find ways we can live together．"28．Mountain lions are moving probably becauseC．A．the suburbs of big cities are peaceful and quietB．more roads are being built into the mountainsC．the number of the lions increases in the westD．young lions are seeking new partners29 According to the passage，it is clear thatB．A．the reported sightings of mountain lions have not been provedB．people walking in the country may come across a lionC．fences have been built to protect the lionsD．mountain lions prefer populated areas30．To avoid attracting mountain lions，people are advisedB．A．to build more roads for the lions to move safelyB．to keep a close watch on their farm animalsC．not to stay outdoorsD．not to raise pets31．Which is the best title for the passage？AA．Mountain Lions in Unlikely PlacesB．Measures Taken to Avoid LionsC．Number of Sightings Going UpD．Mountain Lions Moving West、