It had rained for days; the town was flooded.

Explanation

The sentence "It had rained for days; the town was flooded" is a compound sentence because it:- Has two independent clauses (It had rained for days / the town was flooded)- The two clauses are joined by a semicolon (;)Both clauses are complete sentences that could stand alone, and they are joined together using a semicolon to show that they are closely related in meaning.