Why is the 'o' in 'cold' longer than in 'cot'?

为什么'cold'中的'o'比'cot'中的长？

同一单元的其他句子

• The long vowel sound in 'sheep' is different from 'ship'.单词'sheep'中的长元音与'ship'中的不同。
• Please speak slowly so I can hear the long 'e' clearly.请慢慢说，这样我能清楚听到长音'e'。
• Do you know the difference between 'green' and 'grin'?你知道'green'和'grin'的区别吗？
• The teacher wrote the long vowel words on the board.老师在黑板上写了长元音单词。
• Can you hear the long 'a' in the word 'name'?你能听到单词'name'中的长音'a'吗？