25Verbs such as sing and sink form their past tense by changing the vowel (i.e., from i to a). The term is often used for vowel alternations that mark grammatical contrasts in this way.
(A) ablaut
(B) umlaut
(C) suppletion
(D) reduplication

*五樓指的應該是A選項的 ablaut喔！

(B) umlaut (德文中有點點的母音變形)

Umlaut is a form of assimilation, the process of one speech sound becoming more similar to a nearby sound. If a word has two vowels, one far back in the mouth and the other far forward, it takes more effort to pronounce. If the vowels were closer together, it would take less effort. Thus, one way the language may change is that these two vowels get drawn closer together.

In the general sense, umlaut is essentially the same as regressive metaphony.

有點不懂這段指的是甚麼?

但是例子是如五樓所指的嗎? 不確定喔~請問有人懂嗎? 可以舉例嗎?