Numbers 31 to 33 refer to the following passage. Choose the most appropriate alternative.

Passage 1

Over this decade, employment in jobs requiring education beyond a high school diploma will grow more rapidly than employment in jobs that do not; of the 30 fastest growing occupations, more than half require post-secondary education. With the average earnings of college graduates at a level that is twice as high as that of workers with only a high school diploma, higher education is now the clearest (31)____ into the middle class.
In higher education, the U.S.  has been outpaced internationally. While the United States ranks ninth in the world in the proportion of young adults enrolled in college, we have fallen to 16th in the world in our share of certificates and degrees awarded to adults ages 25-34 lagging behind Korea, Canada, Japan, and other nations. While more than half of college students graduate within six years, the (32) ____ for low-income students is around 25 percent.
Acknowledging these factors early in his administration, President Obama challenged every American to commit to at least one year of higher education or post-secondary training. (33) ____ that America would once again have the highest proportion of college graduates in the world by 2020.

(www. whitehouse.gov)

31. The option that best completes (31) is ....