Why is..?

Excuse me, why the answer to number 10 is ‘A’?

Excuse me, why is this grammatically incorrect? Here are some correct sentences:

Positive
The answer to number 10 is ‘A’.
Negative
The answer to number 10 is not ‘A’.
Yes/No
Is the answer to number 10 ‘A’?
Why
Why is the answer to number 10 ‘A’?
Why (neg)
Why isn’t the answer to number 10 ‘B’?

How about I show you study the correct sentences and then explain the rule in your own words in the comments below?