Mark the letter A, B, C or D to indicate the sentence that is best written from the words/phrases given.Câu hỏi I see/ walk alone/park/ hour ago. A. I have seen him walking alone in the park an hour ago. B. I saw him walking alone in the park an hour ago. C. I saw walking alone in the park an hour ago. D. I have seen him walk alone in the park an hour ago.