下列实验操作中，错误的是（ ）
A．配制5%食盐溶液时，将称量的食盐放入烧杯中加适量的水搅拌溶解
B．实验室里需用490mL0.1mol/L硫酸铜溶液，须称取12.5g胆矾配成500mL溶液
C．测定未知NaOH溶液浓度时，酸式滴定管需用标准酸液润洗2～3次
D．配制0.1mol/L的H2SO4溶液时，将量取的浓H2SO4放入容量瓶中加水稀释
答案：D
If Barack Obama's wife and kids thought they'd be getting his undivided attention during their long-awaited summer holiday, perhaps they should think again. The US President kicked off his vacation by revealing that, in addition to endless games of tennis and golf, he plans to spend the week ploughing through five books, weighing in at an astonishing 2,300 pages. His summer reading list, unveiled(透露) by the White House, contains two heavyweight works of non–fiction and three novels. On top of the president’s table is Hot, Flat and Crowded, by New York Times columnist, Thomas Friedman. Subtitled "why we need a green revolution", it makes a leftish（左倾的） call to arms regarding the future of the planet. Mr Obama's second choice is historian David McCullough's biography of John Adams, the often under-rated second US president, who was the subject of an award-winning HBO docu-drama（纪实剧）last year. The novels include two crime thrillers: Richard Price's Lush Life, and The Way Home, a novel by George Pelecanos set in Washington, DC – which, much like Obama's best-selling autobiography（自传）, explores the relationship between a father and his son. Completing the set is the novel Plainsong, by a little-known writer called Kent Haruf. Set in a small town on the Colorado plains, its existence on the reading list may reassure voters that Middle America has not been ignored by their commander-in-chief. Publishers are keeping an eye on whether the famous "Obama bounce" – which has helped sales at the first family's favourite clothes stores, such as J Crew – will continue to apply to their troubled industry. The President's endorsement（认可） is said to have lifted sales of Joseph O'Neill's novel Netherland about cricket in Holland and New York last year. Given that President Obama has already spent a portion of his week so far playing golf, beating Michelle at tennis, and visiting friends, questions will inevitably(不可避免地) be asked about his ability to put any dent（挫伤） at all in the ambitious reading list. To finish all five books, he would have to manage more than 300 pages every day – quite an "ask" when a small portion of his time must also be spent running the country. 57. What’s the function of the first paragraph?
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