sequence n. [数][计] 序列；顺序；续发事件 vt. 按顺序排好
例句：We call these first, second, third ionization energies in sequence. This is the first ionization energy.
句意：我们按照顺序把这些叫做一级，二级， 三级电离能，这是初级电离能。
故本题选C。