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【多选题】
下列系统中属于非线性系统的有().A.
y(n)=x(n)x(n)
B.
y(n)=2x(n)+1
C.
y(n)=x(n-5)
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【单选题】短周期元素A、B、C、D在周期表中的位置如图所示，若A的原子序数为a、最外层电子数为n，则下列关系式或说法错误的是（ ） A B C D
A.
四种元素的原子序数之和Z=4a+16
B.
四种元素的原子序数之和Z≤50
C.
四种元素的最外层电子数之和E=4n+2
D.
四种元素的最外层电子数之和E≤30
【判断题】天然气和空气混合，当天然气浓度在一定范围内时，遇明火就会发生燃烧和爆炸。燃烧时，燃烧波的传播速度较慢，约0.3～2.4m/s，而爆炸时，会迅即产生高压高温，波速达到1000～3000m/s，对管线和容器的破坏力很大。天然气产生爆炸的浓度范围为5～15％，随温度、压力升高，爆炸浓度的上限提高，爆炸范围扩大。Natural gas and air mix, when the concentration...
A.
正确
B.
错误
【简答题】某种商品在30天内每件的销售价格P（元）与时间t（t∈N * ）（天）的函数关系用如图的两条线段表示，该商品在30天内日销售量Q（件）与时间t（t∈N * ）（天）之间的关系如下表： 第1天 5 15 20 30 Q件 35 25 20 10 （Ⅰ）根据提供的图象，写出该商品每件的销售价格P与时间t的函数关系； （Ⅱ）根据表中提供的数据，确定日销售量Q与时间t的一个函数关系式； （Ⅲ）求该商品的日...
【单选题】One of the most hazardous conditions a fire- fighter will ever encounter is a backdraft (also known as a smoke explosion). A backdraft can occur in the hot-smoldering phase of a fire when burning is i...
A.
has more oxygen available for combustion.
B.
has more carbon dioxide available for consumption.
C.
produces more dense gray smoke.
D.
is more likely to cause a backdraft.
【单选题】One of the most hazardous conditions a fire- fighter will ever encounter is a backdraft (also known as a smoke explosion). A backdraft can occur in the hot-smoldering phase of a fire when burning is i...
A.
carry an oxygen tank.
B.
open a door to allow gases to escape.
C.
make an opening at the top of the building.
D.
break a window to release carbon particles.
【简答题】Fuel gases are very combustible and can easily explode. Explosions can occur when a gas leaks from its container and reaches an ignition source, such as an open flame or spark. In the United States, m...
【单选题】A: 'Bye, darling. Have a good trip. I'll miss you. B: I'll miss you, too. I'll call you every day.
A.
public space
B.
social space
C.
personal space
D.
intimate space
【单选题】One of the most hazardous conditions a fire- fighter will ever encounter is a backdraft (also known as a smoke explosion). A backdraft can occur in the hot-smoldering phase of a fire when burning is i...
A.
windows stained with smoke
B.
flames shooting up from the building
C.
puffs of smoke leaving the building
D.
more intense heat than usual
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