Experts are concerned about the Internet and the troubles (关系代词that/which省略）it has caused to students.

参考译文(1)：互联网令/使专家们很担心以及它给学生们带来的麻烦。

参考译文(2)：专家们对互联网很担心以及它给学生们带来的麻烦。

A. which      B.it      C. what      D that 

(1)concern sb/sth=sb/sth be concerned

(2)concern oneself with sth/sb/doing sth

(3)be concerned about 

(4)be concerned for 

(5)be concerned with

以上5种表达都是正确

concern 用法如下

【1】concern vt.使担忧;使心烦 （英文解释：If something concerns you, it worries you.）（A concern B; A be concerned about/for sb/sth)

The growing number of people seeking refuge in Thailand is beginning to concern Western aid agencies.

到泰国寻求庇护的人日益增多，这开始引起了西方援助机构的不安。

It concerned her that Bess was developing a crush on Max.  

贝丝迷上了马克斯，这让她很担心。

I've been concerned about you lately.

最近我对你有些担心。

We're naturally concerned for our daughter's safety.

我们担心女儿的安全，这很自然。

Academics and employers are deeply concerned that students are not sufficiently prepared mathematically for university courses. 

学生掌握的数学知识不足以应付大学课程，这让大学老师和用人单位非常担心。

【2】concern vt.关心;关注 （英文解释：If you concern yourself with something, you give it attention because you think that it is important.）

I didn't concern myself with politics.(concern oneself with sth)

我不关心政治。

He would concern himself solely with the plight of the hostages.  

他唯一关心的是人质的危险处境。

The agency is more concerned with making arty ads than understanding its clients' businesses. 

这家广告公司更注重如何制作附庸风雅的广告，而忽视了对客户业务的了解。

【3】concern vt.关于;涉及 （英文解释If something such as a book or a piece of information concerns a particular subject, it is about that subject.）

The bulk of the book concerns Sandy's two middle-aged children.

书中主要描写桑迪的两个已到中年的孩子。

Chapter 2 concerns itself with the methodological difficulties.  

第二章讲的是研究方法上的难点。

Randolph's work was exclusively concerned with the effects of pollution on health. 

伦道夫的著作专门论述污染对健康的影响。

【4】concern vt.(情况、事件或活动)对…有影响，与…相关 （英文解释：If a situation, event, or activity concerns you, it affects or involves you.）

It was just a little unfinished business from my past, and it doesn't concern you at all. 

这只是我过去的一点未了之事，与你没有任何关系。

It's a very stressful situation for everyone concerned.

对所有相关人士来说，这一形势非常紧张。

I believe he was concerned in all those matters you mention. 

我认为你提到的那些事都与他有关。