why

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 question
Is the answer to number 10 ‘A’?
Why question
Why is the answer to number 10 ‘A’?
Negative why question
Why isn’t the answer to number 10 ‘B’?

Study these sentences and then explain the rule in your own words to a friend.