阅读下面材料，在空白处填入适当的内容 （一个单词）或括号内单纯的正确形式。
John Snow was a famous doctor in London — so expert that he attended Queen Victoria as her personal physician. But he became inspired when he thought about helping ordinary people ____ (expose) to cholera,____ was the deadly disease of its day. Neither its cause nor its cure was understood. So many thousands of terrified people died whenever there was ____outbreak. John Snow wanted to face the challenge and solve this problem. He knew that cholera would never ____(control) until its cause was found.
He became interested in two theories that ____(possible) explained how cholera killed people. The first suggested that cholera multiplied in the air. A cloud of dangerous gas floated around until it found its victims. The second suggested that people absorbed this disease ____ their bodies with their meals. From the stomach the disease quickly attacked the body and soon the ____(affect) people died. He suspected the second theory was correct ____ he needed evidence.
So when another outbreak hit London in 1854, he was ready to begin his ____(enquire). As the disease spread quickly through poor neighborhoods, he began to gather information. In two particular streets, the cholera outbreak was so severe that more than 500 people died in ten days. He determined ____(find) out why.
