...A...river...B...near...B...under选择不同类的

当然是A了,river是名词，其它的都是介词

river

n.[C]
1. 江,河
2. 巨流;大量[(+of)]
He drinks rivers of tea every day.
他每天喝大量茶水。

near

a.
1. 近的
The post office is quite near.
邮局很近。
2. (关系、程度上)接近的;近似的
Her aunt is her nearest relative.
她姨妈是她最近的亲戚。
3. (两者中)较近的[Z][B]
the near bank of the river
较近的一边河岸
4. 几乎的,险乎的[B]
She wore an expression of near fear.
她面露几近恐惧的神色。
ad.
1. 近,接近
He lives quite near.
他住得很近。
2. 几乎,差不多
It is near impossible.
那是几乎不可能的。
prep.
1. 在...附近
He sat in a chair near the door.
他坐在靠门的一把椅子上。
2. 近似于

under

prep.
1. 在...下面,在...下方;在...里面
What are you wearing under your coat?
你外套里面穿什么?
They were playing chess under the tree.
他们在树下下棋。
2. 少于;低于;未满
The truck was traveling under fifty miles per hour.
那辆卡车以每小时不到五十英里的速度行驶着。
3. (表从属)在...的管理(或统治、领导、监督)之下
We work und