to improve, improve, to improving; listen, to listen, listening
The most effective way______ your listening ability is ______to English radio programs.
【正确答案】：to improve/to listen
【题目解析】：当名词短语前由序数词(the first, the second, etc.)、一般序数词(the last, the next)、最高级的形容词(the best, the most...，etc)或the only修饰时，不定式可用作名词短语的后置修饰语。
