问题 选择题

同温同压下两个容积相等的贮气瓶,一个装有CO,一个装有N2和C2H4的混合气体,两瓶气体一定不相同的是:

A.原子总数

B.总质量

C.分子总数

D.密度

答案

答案:A

根据阿伏加德罗定律可知,两容器具有体积和分子数。由于三种气体的摩尔质量都是相等的,所以密度和质量也是相等的。由于组成分子的原子数是不同的,所以原子数不同,答案选A。

单项选择题

Families separated by the Florida Straits were overjoyed Tuesday and Miami-based charter companies rushed to add seats and flights to their Havana routes after the Obama administration decided to let more Americans visit Cuba as often as they want, and spend as much as they like.

The policy changes are sure to put more money into Cuba’s dying economy, and many would-be travelers are already call for still more barriers to fall. Even Fidel Castro (菲德尔·卡斯特罗) , who had belittled the changes Monday, yielded Tuesday that they were "positive, although minimal".

The United States still doesn’t let most Americans without ties to Cuba visit the island or send in money. But the change, announced Monday by the Obama administration, was among the most significant openings in the half-century of hostilities between the two countries.

During the Bush administration, Americans with immediate family members on the island could visit once every three years and send up to $100 a month to their families.

Now, there are no limits to travel or expense, and the definition of family is expanded to include relatives as distant as second cousins, as well as anyone else who lives as "family" in the home of the authorized traveler.

According to the article, what will this change in policy do for Cuba()

A. It will boost the Cuban economy.

B. It will help defeat Fidel Castro.

C. It will make more Cubans come to the United States.

D. It will decrease the number of Cubans coming to the United States.

单项选择题