Through painstaking effort, they at last worked out a way to____

Through painstaking effort, they at last worked out a way to________ that substance.（pure）
【正确答案】：purify
【题目解析】：原句的意思是“通过艰苦的努力他们最后找到了净化该物质的方法。”因此这里需要填的是个动词，我们填purify。