（二）在铁路、城市轨道交通线路上放置障碍物，或者故意向列车投掷物品的；
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"But by proving the technology it really opens the door for an economically viable product, where things can be made in space and return to Earth and have use and benefit to everybody on Earth. And that's really exciting."
Both page table entries and segment descriptors have an Accessed bit that the hardware must set on use -- but the mechanisms are quite different.