while中間用什麼介詞|for

中間不加介詞，加冠詞a，例如：

1、She'd been seeing the boy for a while， but didn't want her parents to find out.她和這個男孩約會已有一段時間了，但不想讓父母知道。

2、We travelled around for a while and landed up in Seattle.我們到處旅遊了一段時間，最後抵達西雅圖。

3、For a while， she lost the use of her limbs.好一會兒她四肢都動彈不得。