Which is correct? O A. Many cities had to cut down great numbers of stately old elm trees, when they became infected with Dutch elm disease.
O B. Many cities had to cut down great numbers of stately old elm trees. When they became infected with Dutch elm disease.
O C. Many cities had to cut down great numbers of stately old elm trees when they became infected with Dutch elm disease.

O C. Many cities had to cut down great numbers of stately old elm trees when they became infected with Dutch elm disease.

Explanation:

You usually only use commas in between two independent clauses. An independent clause is a sentence that works on its own. It does not need any  more information to understand. It doesn't depend on anything other than itself.

However, you don't need a comma in between an independent clause and a dependent clause. Dependent clauses are just what they sound like: they depend on independent clauses. Dependent clauses are all the background information that is not entirely necessary, but they add detail.

Let's take a look at the other answers:

O A."Many cities had to cut down great numbers of stately old elm trees" is a dependent clause. You can just slap a period at the end it it works just fine. However, there is also a dependent clause with a comma that follows it: ",when they became infected with Dutch elm disease." Remember, you don't follow an independent clause with a comma and a dependent clause. This is a wrong answer.

O B. "Many cities had to cut down great numbers of stately old elm trees. When they became infected with Dutch elm disease." The first sentence is an independent clause, and it works fine. But, the second sentence is a dangling modifier. It describes something, but doesn't show the thing in that sentence. It doesn't make sense.

O C. "Many cities had to cut down great numbers of stately old elm trees when they became infected with Dutch elm disease." This makes perfect sense. It has an independent clause "Many cities had to cut down great numbers of stately old elm trees" and a dependent clause (which needs the independent clause in a sentence with it to work): when they became infected with Dutch elm disease." It doesn't have a comma in between the independent and dependent clauses, so this is the right answer.

pre-writing

why we feel good, or better, when we laugh?

is there a scientific explanation for this to happen?

which are the sources?

rough draft

laughter has a way of instantly connecting people and is one of the most basic and fundamental ways in which we communicate as human beings.

newport academy: endorphins reduce stress and strength social connections.

they combat anxiety and depression.

final draft

we know that when we laugh we feel better; when something is making us laugh it means we thought it was funny at some point, even if it was not meant that way. what we know is that it made us feel good. that is something we all can realize easily, but is it a chemical explanation for it to happen? of course there

according to joe bluett, laughter has a way of instantly connecting people and is one of the most basic and fundamental ways in which we communicate as human beings. also, he says that laughter can increase confidence, self-esteem, creativity, positivity and resilience, bringing positive changes to all aspects of our lives.

the newport academy says that, when we laugh, we release endorphins and that makes our body feel instantly relieved from tension and worry. endorphins also reduce stress and strength social connections.

therefor, laughter is the best to combat anxiety and depression.

i do not the question is put in the corrorect order