题干

选择下列部分实验操作可完成两个实验,甲实验为除去粗盐中难溶性的杂质,乙实验为配制溶质质量分数为10%的氯化钠溶液.下列说法正确的是(  )

A:操作①和⑤中玻璃棒的作用是相同的

B:甲实验和乙实验都要用到的实验操作①

C:甲实验在操作⑤时,将水全部蒸发后停止加热

D:乙实验在操作③时,若俯视读数,会使所配制溶液的溶质质量分数偏小

上一题 下一题 0.0难度 选择题 更新时间:2018-11-26 05:10:09

答案(点此获取答案解析)

B

同类题5

阅读理解

    Chokwe Selassie, aged 13, is working hard to help drivers avoid potholes(坑洼). The eighth-grader was inspired to kick off his creation on a recent morning, when his mother was driving him to school. Their car was damaged as it went over a huge pothole in the middle of the street in their hometown of Jackson, Mississippi. He decided to do something about the pothole problem in his city. His solution: an app that warns drivers when there is a pothole ahead.

    Chokwe developed the app with his friends Rodriguez Ratliff and Emmanuel Brooks. When the app detects a pothole, it is highlighted in red. And if you get close to the pothole, your phone will warn you. Drivers can also use the app to report any potholes they meet, and to look for other routes they can take to avoid roads that have them.

    The app relies on current available information about the streets of Jackson. It works by using the city's 311 call system, and uses information already stored in a database. Through the call system, citizens dial 311 to report non-emergency problems, which include potholes. Chokwe and his friends determined that focusing on the 10 busiest streets in Jackson would give them a large enough sample size to test the prototype(原型).

    Although the app isn't yet available for sale,Chokwe is already looking for ways to improve it. The prototype remains limited to 10 streets in Jackson, but he hopes to add more, so that it includes every street in the city. And then he wants to go even farther until it's nationwide.