搜题请搜索小程序“爱下问搜题”
问题 填空题

饲料中钙的测定（高锰酸钾法）的原理是将分解的试样与草酸铵溶液煮沸生成（），为使沉淀陈化必须将试样（）。

答案

参考答案：草酸钙沉淀；放置过夜

单项选择题

检查气瓶压力及整机气密性。打开节气开关，逆时针方向旋转瓶阀手轮2圈，气体应从供气阀中流出，关闭节气开关，气流应停止。观察压力表的读数，气瓶内空气压力应不小于24兆帕（MPa）。顺时针旋转瓶阀手轮，关闭瓶阀，继续观察压力表读数（）分钟，压力降低不超过2兆帕（MPa），则系统气密性良好。

A.1

B.2

C.3

D.4
查看答案
单项选择题

Questions 6～10


It is Monday morning, and you are having trouble waking your teenagers. You are not alone. Indeed, each morning, few of the country’s 17 million high school students are awake enough to get much out of their first class, particularly if it starts before 8 am. Sure, many of them stayed up too late the night before, but not because they wanted to.
Research shows that teenagers’ body clocks are set to as schedule that is different from that of younger children or adults. This prevents adolescents from dropping off until around 11 pm, when they produce the sleep-inducing hormone melatonin, and waking up much before 8 am when their bodies stop producing melatonin.
The result is that the first class of the morning is often a waste, with as many as 28 percent of students falling asleep; according to a National Sleep foundation poll. Some are so sleepy they do not even show up, contributing to failure and dropout rates.
Here is an idea: stop focusing on testing and instead support changing the hours of the school day, starting it later for teenagers and ending it later for all children. Indeed, no one does well when they are sleep-deprived, but insufficient sleep among children has been linked to obesity and to learning issues like attention deficit/hyperactivity disorder. You would think this would spur educators to take action, and a few have.
In 2002, high schools in Jessamine County in Kentucky pushed back the first bell to 8:40 am, from 7:30 am. Attendance immediately went up, as did scores on standardized tests, which have continued to rise each year. In Minneapolis and Edina, Minnesota, which instituted high school start times of 8:40 am and 8:30 am respectively in 1997, students’ grades rose slightly and lateness, behavioral problems and dropout rates decreases. Later is also safer. When high schools in Fayette County in Kentucky delayed their start times to 8:30 am, the number of teenagers involved in car crashes dropped, even as they rose in the state.
So why has not every school board moved back that first bell Well, it seems that improving teenagers’ performance takes a back seat to more pressing concerns: the cost of additional bus service, the difficulty of adjusting after school activity schedules and the inconvenience to teachers and parents.
But few of these problems actually come to pass, according to the Center for Applied Research and Educational Improvement at the University of Minnesota. In Kentucky and Minnesota, simply flipping the starting times for the elementary and high schools meant no extra cost for buses.
There are other reasons to start and end school at a later time. According to Paul Reville, a professor of education policy at Harvard and chairman of the Massachusetts Board of Education, "trying to cram everything out 21th-century students need into a 19th-century six-and-a-half-hour day just isn’t working". He said that children learn more at a less frantic pace, and that lengthening the school day would help "close the achievement gap between disadvantaged students and their better-off peers".

According to the passage, what determines a person’s body clock is ______.

A. melatonin production
B. one’s lifestyle
C. schedule setting
D. one’s sleep patterns
查看答案