如图所示，△OAC和△BAD都是等腰直角三角形，∠ACO=∠ADB=90°，反比例函数 $\text{y=}\frac{\text{k}}{\text{x}}$ 在第一象限的图像经过点B，与OA交于点P，若OA²-AB²=18,则点P的横坐标为（     ）

植物细胞有而动物细胞没有的能量转换器是（　　）

    Sam's parents took her to the World Park last week. He saw Big Ben, the Tower Bridge, Disneyland in America, the CN Tower, Niagara Falls and the Sydney Opera House. His father told her Big Ben and the Tower Bridge are in London. Her mother told her Niagara Falls is in Canada and the Sydney Opera House is in Australia. He learnt about many things of the famous places around the world.