With a coordinator: He heard someone knock at the door violently

With a coordinator: He heard someone knock at the door violently______he looked out of the window only to find that his house was surrounded by a large crowd of policemen.
【正确答案】：and
【题目解析】：原句的意思是“他听到有人猛烈地敲门，他望向窗外结果发现一大群警察把房子围了起来。”