某溶液的pH值约为6，用酸度计测定其精密pH值时，应选择的两个标准缓冲液的pH值是（）
A.1.68，4.00
B.5.00，6.86
C.4.00，6.86
D.6.86，9.18
E.1.68，6.86
参考答案：C
解析：校正时应选择二种pH值相差3个单位的标准缓冲溶液，使供试品溶液的pH值处于二者之间，某溶液的pH值约为6，所以范围为3～9。
| 阅读短文，根据短文内容，从短文后的五个选项中能出选填入空白处的最佳选项。有一个选项为多余选项。 | |
| Mabel Welton, 62, was in her living room when a piece of dark gray metal hit the roof of her house. 1 . Because her house in Hessle , UK lies under a flight path. However, after examining the piece, the Royal Air Force( RAF) said that the metal was space junk that fell to Earth. 2 . "We heard this sound and went outside and saw this hole in the roof, " she said. " The metal had broken two tiles and made a mess. " Welton said that her granddaughter's boyfriend Richard Sutcliffe had to＂ put on some oven gloves to get it because it was so hot. " The Weltons gave the metal to the police. 3 . Last Thursday, the RAF said they learned that the metal was space rubbish. 4 . A great number of pieces from more than 5,000 old satellites are now in space. Most pieces have fallen to Earth without doing damage to people or houses.
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