fly in the sea还是fly on the sea?

fly in the sea还是fly on the sea?
There are lots of birds flying _____ the sea.
A.on B.over C.in D.through
就是我看其他几个答案也对的~`郁闷！有什么区别啊？

fly in the sea
词 at,on,in at指一个“点”或“小地方”；
in指一个“地区”或“大地方”空间内；on在某一平面或线上面.They arrived at the village at ten.
Your pencil is in the desk.
Your pencil is on the desk.
on,above,over on“与物体接触”反义词为“beneath”；over“在…正上方”反义为“under”；above“在…的上方”反义词为“below”.There’s a book on the desk and a pen beneath the book.
There’s a lamp over the desk and a bag under the table.
Jack’s bedroom is above mine on the second floor.
over,across,through across“表面跨过”；
through“从中间或从头至尾穿过”；
over“从上空越过”.He’s swimming across the river.
They walked through the forest.
A plane is flying over the city.
at,beside,by,next to,near at靠得“最近”；
beside“在并排一条线上”；
by“在并排一条线上”；
next to“在顺序上紧靠旁边”；
near 靠得“最远”,不表明确方向或顺序.Let’s meet at the school gate.
The building beside the library is the shopping center.
The boy standing by my side is from Shanghai.
Who’s sitting next to/beside Mr.Steve?
about,round,around.about表示周围是随意的,不规则的；round或around(二者无多大差别)的周围则是较完整的一个圈.Don’t leave the toys about the meeting-room.
They are sitting round/around the table.
to,for,at to “运动的方向,目的地”；
for “动身出发的目的地”；
at “有意攻击的目标”.Throw it to me.
He’ll leave for Shanghai.
He threw the ball at that boy.
up,down up指“往上,往北,大地方,靠拢”；
down指“向下,往南,向小地方,往开走”但在一市区,去中心区用down,去郊区等用up.They are going up the hill.
They are going down river.
I’m going down town shopping.
on,in,to in表“在某地区内”；
on表“接壤”；
to表“在某地区以外”.Shanghai is in the south of China.
Hunan lies on the north of Guangdong.
Xichang lies to the south west of Chengdu.
between,among between指“两者之间”；
among指“在三者以上之间”.There’s a river between the two villages.
There’s a small house among the trees.
besides
except
except for
except that
but
but for besides“除…之外还有,”实际不排除；
except“除…外”表“排除”,不放在句首；
except for表整体肯定补充细节,表除去整体中的一部分；
except that“除了…外”,后接从句；
but通常与all,no,every,where,who what及有些它们的合成词连用；but for“要不是”,后面句子常用虚拟语气.I love music besides sports.
The house is never used except in winter.
The bus is empty except for an old woman.
I don’t know Peter except that he’s an Japanese.
There’s nothing but a chair in the room.
But for your help,I wouldn’t have finished my homework