It is found that the fundamental reason why polo was just popularized in upper class society is that only those who were prosperous and unoccupied could have enough money for this kind of highly standardized and consumed sport at that time when the feudal economy was flourishing .

结论：唐代马球运动流行于上层社会的根本原因，在于在 当时封建经济发达的背景下，唯有富裕而有 闲 的 上层社会才具备开展高标准、高消费的马球运动的经济实力。

The house was unoccupied at the time of the explosion

爆炸发生的 时候房子里 没有 人 。