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当前学科：大数据开发技术
题目：
未知类型
Spark中DataFrame的()方法是进行连接查询。
A.where
B.join
C.limit
D.apply
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新闻批评
A Bridge for Squirrels San Francisco has its cable cars. Seattle has its Space Needle. And， Longview has its squirrel bridge. The bridge， which has attracted international attention， is now a local landmark. The Nutty Narrows Bridge was built in 1963 by a local builder， Amos Peters， to give squirrels a way to cross the busy road without getting flattened by passing cars. The original bridge was built over Olympia Way on the west edge of the library grounds. Before the bridge was built， squirrels had to avoid traffic to and from the Park Plaza office building where office staff put out a nutty feast for the squirrels. Many times， Peters and others who worked in and near Park Plaza witnessed squirrels being run over. One day Peters found a dead squirrel with a nut still in its mouth， and that day's coffee break discussion turned into squirrel safety. The group of businessmen cooked up the squirrel bridge idea and formed a committee to ask the blessing of the City Council （市政会）.The Council approved， and Councilwoman Bess LaRiviere jokingly named the bridge "Nutty Narrows". After architects designed the bridge， Amos Peters and Bill Hutch started construction. They built the 60-foot bridge from aluminum and lengths of fire hose （消防水带）. It cost 1000. It didn't take long before reports of squirrels using the bridge started. Squirrels were even seen guiding their young and teaching them the ropes. The story was picked up by the media， and Nutty Narrows became known in newspapers all over the world. In 1983， after 20 years of use， Peters took down the worn-out bridge. Repairs were made and crosspieces were replaced. The faded sign was repainted and in July 1983， hundreds of animal lovers attended the completion ceremony of the new bridge. Peters died in 1984 ，and a ten-foot wooden squirrel sculpture was placed near the bridge in memory of its builder and his devotion to the project. What does the underlined phrase "teaching them the ropes" probably mean in the text？（）