25.试题分析：考查定语从句。本题中的介词to是从动词return后面移至关系代词which前面。句意：风能是一种古老的能量来源，在不久的未来我们就会重新使用它。Which指代前面的先行词an ancient source of energy作为介词to的宾语。动词return与介词to连用，故C正确。

26. in是和the way搭配的

另外定语从句that前面不可以有介词

你可以较为全面的再去补充语法点

这是刘永科老师的一个总结：

“介词 + 关系代词”结构讲解：

“介词 + which”就是把介词提到关系代词which 之前，它是定语从句中常见的语言现象，也是一些网友不易掌握的一个语法项目，问题的关键是这个“介词”较难确定。但它是有规律可循的，有以下几种情况，详解如下：

1. “介词 + which”可以替代关系副词when, where, why

(1)替代when

I'll never forget the day on which (=when) my son was born. 我永远忘不了我儿子出生的那一天。（on依据on the day确定）

The year in which (=when) the earthquake happened was 2008. 发生地震的那年是2008年。（in依据in the year确定）

(2) 替代where

This is the office in which (=where) I used to work.这就是我过去工作过的办公室。（in依据in the office确定）

He forgot the exact place in which (=where) he had hidden the jewelry. 他忘记了埋藏珠宝的确切地点。（in依据in the place确定）

(3) 替代why

I'd like you to explain the reason for which (=why) you refused my offer. 我想让你解释一下你拒绝我的帮助的原因。（for依据for the reason确定）

2. 依据与先行词搭配的具体意义确定使用什么介词

I'll never forget the time during which I was with my friends in the country. 我永远不会忘记和朋友们一起在乡村度过的时光。(during依据during the time确定)

I will offer you a better way in which you can do the job. 我要给你提供一个做这项工作较好的办法。(in依据in a better way确定)

3. 依据定语从句中动词或形容词的搭配来确定介词

She bought several clothes, on which she spent all her money. 她买了几件衣服，这些衣服花光了她所有的钱。（on依据spend…on确定）

It is the grammar rules of which I am not sure. 我没有把握的就是语法规则。（of依据be sure of确定）

4. 根据定语从句所表达的具体意思来确定介词

Can you see the river across which there is a bridge? 你能看见上面有座桥的那条河？

He is swimming in the river, on which there is a boat. 他在那条河里游泳，河面上有一只小船。

Here is the river in which I used to swim. 这儿就是我过去常在里面游泳的那条河。

【注意】

有些“动词+介词”短语被视为一个固定的词组，不可分割。这时不能把介词置于关系代词前，如：listen to, look at, look for, look after, care for, hear from, send for,等。

This is the right book for which I'm looking. (误)

This is the right book that I'm looking for. (正)

就是我要找的书。

此外，除了“介词 + which”以外，如果定语从句的先行词是“人”，那么也可以用“介词+whom”引导定语从句，例如：Mary is the girl for whom I bought this gift. 玛丽就是那个我为其买此礼物的女孩。