In 1939 two brothers, Mac and Dick McDonald, started a drive-in restaurant in San Bernadino, California. They care fully chose a busy comer for their location. They had run their own business for years, first a theater, then a barbecue(烤肉) restaurant, then another drive-in. But in their new operation, they offered a new, shortened menu: French fries, hamburgers, and sodas. To this small selection they added one new concept: quick service, no waiters or waitresses, and no tips. Their hamburgers sold for fif cents. Cheese was another four cents. Their French fries and hamburgers had a remarkable uniformity, for the brothers had developed a strict routine for the preparation of their food, and they insisted on their cooks" sticking to their routine. Their new drive-in became incredibly popular, particularly for lunch. People drove up by the hundreds during the busy noontime. The serf-service restaurant was so popular that the brothers had allowed ten copies of their restaurant to be opened. They were content with this modest success until they met Ray Kroc. Kroc was a salesman who met the McDonald brothers in 1954, when he was selling milkshake-mixing machines. He quickly saw the unique appeal of the brothers fast food restaurants and bought the right to franchise(特许经营)other copies of their restaurants. The agreement struck included the right to duplicate the menu. The equipment, even their red and white buildings with the golden arches. Today McDonald"s is really a household name. Its names for its sandwiches have come to mean hamburger in the decades since the day Ray Kroc watched people rush up to order fif-cent hamburgers. In 1976, McDonald"s had over $1 billion in total sales. Its first twenty-two years is one of the most incredible success stories in modem American business history. As used in the second sentence of the third paragraph, the word "unique" means ______.