将三个木板1、2、3固定在墙角，木板与墙壁和地面构成了三个不同的三角形，如图所示，其中1与2底边相同，2和3高度相同，现将一个可以视为质点的物块分别从三个木板的顶端由静止释放，并沿斜面下滑到底端，物块与木板之间的动摩擦因数μ均相同，在这三个过程中，下列说法正确的是(    )

    Volunteers are people who do a job for the satisfaction (满足感)of helping others．

    Volunteers do many different types of jobs．Some volunteers read books onto tapes so that people with poor eyesight（视力）can listen．

    Some volunteers deliver meals to elderly or sick people.

    Some volunteers help tourists (游客) who visit a city during a major event like the Olympic Games or the Commonwealth Games.

    Volunteers help whenever a community is hit by a natural（自然的）disaster（灾害）.

    Volunteers assist（援助，帮助）in putting out（扑灭）bush（灌木丛）fires．They might also help to evacuate（疏散）people from areas that have been flooded or hit by an earthquake（地震）or tsunami（海啸）.

    A community may not recover（恢复正常）without the help of volunteers.

    Some communities require volunteers more than others.

    Communities that don't have much money require a great deal of help from volunteers.

    Communities with a large number of elderly people might require more help from volunteers than communities with more young people．Without volunteers, some communities couldn't operate.