Mr. Black was very forgetful. Each time when Mrs. Black wanted her husband to do something, she had to tell him again and again not to forget it. Once Mr. Black was asked to post a letter, " Do remember to put it into the letter box on your way to work." The wife said. " This time I won't forget." Mr. Black said to his wife. "I will hold the letter in my pocket." "Oh, my dear, you may go now. Please come back early." Mrs. Black said. As Mr. Black was shown out, he felt as if his wife touched him affectionately   on the back. Hand in his pocket, Mr. Black walked slowly down the street. Soon a man caught up with him from behind and smiled at him, "Don't forget to post the letter." Mr. Black was wondering how the man knew he had a letter to post. Then a girl passed him and turned to smile at him, too."Now be sure to post the letter," the girl said. Mr. Black was even more puzzled, "Why are they smiling at me? And how do they know I'm going to post a letter?" he thought to himself. Do remember to put it into the letter box on your way to work showed   _   .
Mrs. Black believed her husband would certainly do as he was told to Mrs. Black believed husband's memory Mrs. Black doubted her husband's memory Mrs. Black hoped her husband wouldn't post the letter in a wrong place