题目

阅读下面的短文，并根据要求回答后面的问题。（注意字数要求）        [1] If you are applying for an American higher education, you probably dream of getting into an Ivy League （常青藤联盟） university．The eight schools with the best reputations attract top students from the United States and many other countries． However, on average, the Ivy League schools accept less than 15 percent of applicants．And with the applicants growing in size year on year, it is becoming more and more difficult _________．        [2] So how can you give yourself t核内中子数为N的R2+离子，质量数为A，则n克它的氧化物所含电子物质的量为（ ）A．B．C．D．