Join the following sentences to make one complete sentence without using and, but or so.
Mumbai is densely populated. It is one of the major cities in the country.
Topic: Synthesis — apposition
Answer
Mumbai, one of the major cities in the country, is densely populated. (Also acceptable: Mumbai, which is one of the major cities in the country, is densely populated.)
The two sentences both describe the same subject, so the neatest join is an APPOSITIVE PHRASE — a noun phrase set between commas that renames the subject.
"Mumbai, ONE OF THE MAJOR CITIES IN THE COUNTRY, is densely populated."
The relative-clause version, "Mumbai, WHICH IS one of the major cities...", is equally correct and slightly heavier. Apposition simply drops the "which is".
THE COMMAS ARE ESSENTIAL and are part of the mark. The phrase adds extra information rather than identifying which Mumbai is meant, so it is non-defining and must be enclosed on both sides. Omitting the second comma is the commonest error.
Apposition is worth knowing because it is the shortest way to join two facts about one subject without any conjunction at all.