Complete the following sentence with a (compound) word derived from the one(s) given in bracket.
Through painstaking effort , they at last worked out a way to （ ） substance. (pure)
【正确答案】：PURIFY
【题目解析】：
根据前文 a way to do 可知空处应该填动词原形。pure的动词原形为purify 使...净化。句意：通过辛苦的努力，他们最终找到净化物质的方法。