In the past ten years, the number of doctorates awarded by our country has continued to rise, so we can consider that the number of graduate students in our country has been increasing in the past ten years. Of the following items, which best supports the above conclusions? ['The difficulty of obtaining a Ph. D. has not changed in the past ten years.', 'In the past ten years, the proportion of the number of doctoral students in our country has remained unchanged.', 'The number of applicants for Ph. D. this year is about twice that of ten years ago.', 'This year, China ’s doctoral students account for only about 20% of graduate students.']