question: For what reasons are elevators routed one-by-one ?
answer: when they reach a pre-determined passenger load , or when they have had their doors opened for a certain period of time
turker_answer: Elevators are routed one-by-one when they reach a pre-determined passenger load or when they have had their doors opened for a certain period of time .
rule-based: Elevators are routed one-by-one for when they reach a pre-determined passenger load , or when they have had their doors opened for a certain period of time .