如图甲所示，放在水平地面上的物体，受到方向不变的水平拉力F的作用，其F﹣t和V﹣t图象分别如乙、丙所示，由图象可知，当t=1s时，物体受到的摩擦力是____N，当t=3s时，物体受到的摩擦力是____N。

    Golden Gate Bridge

    Located in San Francisco,the Golden Gate Bridge started in the year 1933 to connect the San Francisco Peninsula with Marin County.It was finally thrown open to public traffic in 1937.It cost $25.7 million in the construction.Till the year 1957,the Golden Gate Bridge,at a length of 2,737 meters,was the longest suspension bridge in the world.

    Brooklyn Bridge

    The Brooklyn Bridge is located in Brooklyn.It is one of the oldest suspension bridges in the United States, having been opened in the year 1883.The length of the bridge is 1,843 meters.The bridge has been featured in several Hollywood movies.

    George Washington Bridge

    Also known as the Hudson River Bridge and the Columbus Bridge,the George Washington Bridge which connects Fort Lee to Manhattan came into use in 1931 after a construction period of almost 4 years.It is a two level suspension bridge that cost about $52 million to build.

    Mackinac Bridge

    This is the third biggest suspension bridge in the world at a length of 8,038 meters.The architect of this bridge was Dr.David B. Steinman who directed the construction of the bridge which started in the year 1054 and opened to the public in 1958.People using this bridge are charged a certain amount of money.

    Navajo Bridge

    Located in Arizona,this bridge crosses the Colorado River and is almost 250 meters long.The construction of this bridge started in the year 1927,ending two years later,costing $390,000.In the 1990s a second bridge was built which was opened to the public in 1994.The first bridge is now used only by pedestrians.

读下表我国某地区地形类型比例以及土地利用现状图，该地区耕地比重过高，容易造成的环境问题是（    ）。