题干

在化学世界里没有生命的阿拉伯数字也变得鲜活起来,它们在不同的位置表示着不同的含义.下列化学符号中数字“2”表示的意义正确的是(   )

A:Mg2+:一个镁离子带2个单位正电荷

B:CO2:二氧化碳分子中含有氧原子

C:2H:2个氢元素

D:CaO+2 :氧化钙的化合价为+2价

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答案(点此获取答案解析)

A

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Psychology(心理学)tells us that many people hate to take risks. But it is good for us to take risks, especially when the risk is to achieve a desired(渴望的) result. In that way, we become stronger and braver.
Our human nature should be to take risks, but some people just sit and wish they didn’t have the fear to move on. This is because they failed a few times in their lives. Please step out and don’t let the past hold you back from living life to the fullest. Move forward and move on! In studying the psychology of taking risks, we find that human nature provides us with the desire to experiment and take chances.
Risk taking is a great advantage that allowed our ancestors(祖先) to become stronger and stronger day by day. By taking risks they fought off enemies and discovered new territories(领土). This attitude has become a part of our modern culture. Riding a roller coaster is a common risk taking activity. Even each person seems to enjoy the risk although they have the understanding that it is dangerous. This psychological and biological (生物学的) connection creates an interesting connection between what is unsafe and what humans enjoy.
Getting in a car each day is a risk. Getting out of bed is a risk, too. We need to take risks so that we can complete many things. Astronauts take risks when they get inside a rocket(火箭); however, the things they achieve are great. Businessmen take a risk when they buy parts of a company. However, without doing that, they could not make more money.
We need to take risks so that we can gain(获得) something. It is impossible to move forward in life, earn money, enjoy a relationship, play a sport, or do anything else without taking a risk. It is all part of the game. It’s one of the most important parts of life.