A、The number of teachers has decreased. B、The quality of higher education has declined. C、The higher education has expanded mass

In the US, (30)public and policy discussions of higher education over the course of the 20th century have focused on one issue in particular: access. Massive expansion of higher education, led by the public sector, has created unprecedented opportunities for students to continue their education beyond high school. Although institutional barriers and inequalities in access persist and concerns about affordability continue to mount, American higher education today educates more than 18 million students in more than 4 300 degree-granting institutions. Educational expectations have been on the rise, with more than 90% of high school students expecting to attend college. And many are indeed crossing the threshold of higher education: More than 70% of recent high school graduates have enrolled in either a two-year or a four-year institution. (31)As Martin Trow has observed, higher education has been transformed from a privilege into an assumed right, and for a growing proportion of young adults, into an expected obligation. Although growing proportions of high school graduates are entering higher education, many are not prepared for college-level work and many others have no clear plan for the future. (32)Most American high schools have come to embrace a "college for all" mentality, encouraging students to proceed to higher education regardless of their academic performance. Consequently, high school students expect to enroll in college and complete bachelor’s degrees, even when they are poorly prepared to do so judging from their grade point averages, high school rank, or courses taken. Almost half of students agreed with the statement: Even if I do not work hard in high school, I can still make my future plans come true.
30. What’s the main problem of US higher education in the 20th century?
31. What can we conclude from Martin Trow’s observation?
32. What is the consequence of embracing a "college for all" mentality?