A、The enrolment rate is undergoing its record low. B、Poor students cannot afford the high tuition fees. C、Less and less students

Less than 60% of students now entering four-year American college degrees are likely to graduate. The completion rate is lower than that in almost any other wealthy country, and worst for poor and minority students. A new book about America’s public universities explores the complex causes of the high dropout rate. The book is called Crossing the Finish Line.
    President Obama wants the United States to again have the world’s highest percentage of college graduates by 2020. But to finish college, children first have to reach the starting line by getting there.[17]On Tuesday the President gave a nationally broadcast speech to students about the importance of staying in school. He spoke on the first day of classes at a high school in Virginia.[18]He talked about personal responsibility, and used himself as an example of someone who overcame difficulties.
    Barack Obama says, "My father left my family when I was two years old, and I was raised by a single mother who struggled at times to pay the bills and wasn’t always able to give us things the other kids had. There were times when I missed having a father in my life. There were times when I was lonely and felt like I didn’t fit in." But he told students that problems in their own lives should not stop them from learning.
16. What problem do American colleges have now, according to the passage?
17. Why did Obama give a nationally broadcast speech to students?
18. What do we learn about Obama from the passage?