question: What has happened to photovoltaic in the past 20 years ?
answer: evolved from a pure niche market of small scale applications towards becoming a mainstream electricity source
turker_answer: Photo voltaic has evolved from a pure niche market of small scale applications towards becoming a mainstream electricity source in the past 20 years .
rule-based: Evolved from a pure niche market of small scale applications towards becoming a mainstream electricity source has happened to photovoltaic in the past 20 years .