T=teacher,TO=Tommy

T=teacher,TO=Tommy
T:Give me a sentence starting with "I",Tommy.
TO:Yes,sir.I is...
T:No,no,Tommy!Don't say "I is",you should say "I am".
TO:All right,sir.I am the ninth letter of the aiphabet.理论上绝对合理的用法"I am"怎么就突然变成了错误的,而本该错误的“I is”反倒成了合理的呢?
这道题是不是出错了?应该怎么回答?