this or these

[diamondcutter]

Students these days often have a lot of worries. Sometimes they have problems with their schoolwork, and sometimes with their friends. What can they do about this? Some people believe the worst thing is to do nothing.
Source: An English textbook for junior high school students in China, by PEP and Cengage Learning

What does the pronoun “this” refer to? Does it refer to the students’ worries or problems? Why didn’t the writer use the plural pronoun “these”?

 

[Barque]

Does it refer to the students’ worries or problems?

The word "worries" in the first sentence includes the problems with schoolwork and with friends referred to in the second.

The word "This" refers to the fact that they have a lot of worries. (This fact is a singular thing, and needs the word "this".)

It refers to the overall problem that students have (which is that they have a lot of worries, which in turn includes problems with schoolwork and with friends).