ground floor的复数形式怎么写？

n.第一层...

The elevator creaked to a halt at the ground floor.电梯嘎吱一声停在了一层。She showed him around the ground floor of the empty house.她带他参观了这处空房子的底楼。Investigations had revealed extensive rot in the beams under the ground floor.调查发现一层...

ground floor的音标：ground floor的英式发音音标为：[ɡraund flɔ:]ground floor的美式发音音标为：[ɡraʊnd flɔr]...