题目

    It was a rainy day and I was riding on a bus downtown to go to work. I was   36   next to a man in a business suit and I paid little attention  37   we both got off at the same stop and walked to the same newsstand to get a morning   38  .  The man running the stand was obviously having a  39   day. He was rude and unsmiling as we bought our papers, which only  40   more gloom （郁闷）to my day. The businessman   41 my gloomy feeling and smiled brightly,   42   the news stand owner for the paper and for being open on such a（n）   43   to make sure we we某物质中只含有一种元素，则该物质（ ）A．一定是单质B．一定是混合物C．一定是纯净物D．一定不是化合物