saw,seen,were seen;when,that,whoFlight has been the dream of hum

saw,seen,were seen;when,that,who
Flight has been the dream of humankind since birds ________ in the sky.But it wasn't until the 1780s ________ two Frenchmen flew in a hot balloon near Paris.After that,powered flight became the goal.
【正确答案】：were seen;that