once是什么词性

　　例句：

　　We didn't know how we would cope once the money had gone.

　　一旦钱花光了，我们就不知道该怎么办了。

　　She fell in with my idea at once.

　　她立刻同意了我的主意。

　　If you can only exercise once a week, that's better than nothing.

　　即便是一个星期锻炼一次，也比完全不锻炼好。

　　It was once thought that the sun travelled around the earth.

　　人们曾经认为太阳绕着地球转。

　　Let me hear it just once more.

　　让我再听一遍。