食品中氨基酸态氮含量的测定方法有电位滴定法、双指示剂甲醛滴定法，该方法的作用原理是利用氨基酸的两性性质，甲醛的作用是（）。
参考答案：固定氨基
I really appreciate _____ to relax with you on this nice island. [ ]
A. to have had time
B. having time
C. to have time
D. to having time
（）年，中央人民政府正式宣布五月四日为“青年节”。
A.1921
B.1949
C.1953
