fly across 后面的across 是介词还是副词

还有fly alone across 搞不懂。

若 across 后面跟有宾语，它就是介词；若 across 后面没跟宾语，它就是副词。比较：

The mosquito flew across the room.蚊子飞过房间。（介词）

A lone gullflew across the sky. 一只孤单的海鸦从空中飞过。（介词）

With the explosion, glass flew across the room.伴随一声爆炸，房间内玻璃碎片四处横飞。（介词）

A bird flew across to the tree and alighted on a branch.一只鸟飞向对面那棵树，停在一根树枝上。（副词）