foot复数形式是什么

　　Your feet must be behind the line when you serve.

　　发球时你的'脚必须站在底线的外面。

　　She shifted her weight from one foot to the other.

　　她把身体的重量从一只脚换到另一只脚上。

　　It was the first time I had set foot on American soil.

　　那是我第一次踏足美洲大地。

　　You can back up another two feet or so.

　　你可以再退两英尺左右。

　　It was the first time I had set foot on African soil.

　　那是我第一次踏上非洲大地。