语法填空里什么时间用不定冠词和定冠词

语法填空习题里出现了如此一个空：

These are called polymorphic. "polymorphic" means||||||ability to take different forms.

答案给的是 the，请问此处为何不可以填 an。前文也并没出现 ability。

通常来讲语法填空里填定冠词都是前文有出现同样词的，假如填不定冠词，一般是前文没出现过同样的词。但有时会有特殊状况。请问该怎么分辨？

These are called polymorphic."polymorphic" means the ability to take different forms.

参考译文：这类都被叫做多态性的。“多态”意思是具备不同形式的能力。

解答：名词ability后面后不定式“to take different forms”修饰限定，所有要加“定冠词the”表示特指。

像“the ability to do sth=做某事的能力”句子如下

He has the ability to bring out the bestin others.  他能做到扬人之长。