a few years ago前为什么不加介词in

a few years ago前为什么不加介词in every few years 中,few前为什么不加“a”? ten years ago 前加什么介词? in a few years和 after years ago的区别 a few hundred years ago ten minutes ago为什么不加介词 为什么in the last few months 中 few前面不加a A few years ago,there was lots in the last few years 为什么要加 the last weekend前为什么不加介词? in the last few years 省略介词 in 25 to 50 years,有介词,为什么20 years ago之前没有