question: What darkens a conventional bulb over its lifetime ?
answer: the evaporated tungsten eventually condenses on the inner surface of the glass envelope
turker_answer: The evaporated tungsten eventually condenses on the inner surface of the glass envelope and darkens a conventional bulb over its lifetime .
rule-based: The evaporated tungsten eventually condenses on the inner surface of the glass envelope darkens a conventional bulb over its lifetime .