题干

足量的蜡烛在密闭的集气瓶内燃烧至熄灭,用仪器测出瓶内氧气含量的变化如图所示。下列判断正确的是(   )

A:蜡烛燃烧前瓶内只有氧气

B:氧气浓度小于一定值时,蜡烛无法燃烧

C:过程中瓶内物质总质量不断减少

D:蜡烛熄灭后瓶内只剩二氧化碳气体

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答案(点此获取答案解析)

B

同类题4

阅读下面短文,从短文后各题所给四个选项(A、B、C和D)中,选出可以填入空白处的最佳选项,并在答题卡上将该项涂黑。

    A new term has begun. Teachers are worried about the fact that new students are not easy to deal with. They like to bring cell phones and MP3 players to school. What is 1, some students even use cell phones 2out-of-class matters in class, or sometimes just for fun. Some 3 students listen to MP3 players when they are having a lesson that they are not interest in.

    Are these new students really that 4? “Yes.” says Delaney Kirk, a professor at Drake University. 5 she adds it's not their fault. 6, the teachers should be blamed(责备). Mrs Kirk first began thinking about students' manners six years ago. “I had my first class in which students were sleeping or talking to each other. It seemed that learning well or had nothing to do with them.” she says. “At first, I got worried about this.” But then I said to myself, “You're giving lessons, and you need to manage this kind of situation. These students need to know more about manners. It's time to help them develop some good 7. They shouldn't waste time doing nothing when they are young. Sooner or later, they may regret the time they8.”

    Mrs Kirk also 9a list of suggestions to help teachers better manage their classes. The following are among her suggestions:

●On the first day of class, tell students how they will benefit(受益) by taking the class and the importance of listening carefully in class.

●Do not allow them to bring cell phones or MP3 players to the class at all.

● Tell them how to use cell phones or MP3 players 10.