Read the sentence below to find out whether there is any grammatical error in it. The error, if any, will be in one part of the sentence. Choose the option with that part as your answer. If there is no error, choose 'D' as your answer. (Ignore the errors of punctuation, if any).
While walking slowly in the park on a quiet summer afternoon, a mad dog suddenly attacked him from behind
While walking slowly in the park
on a quiet summer afternoon,
a mad dog suddenly attacked him from behind
No error
Correct answer is A
While he was walking slowly in the park