Question

Is the sentence a simple or a compound sentence?

Stone bridges are strong but are costly to build.
a. compound sentence
b. simple sentence

Answer 1

This is a compound sentence, because there are two independent clauses here: 1. Stone bridges are strong + but 2. (strong bridges) are costly to build.
The conjunction 'but' connects two independent clauses, and a compound sentence contains at least two independent clauses, whereas a simple sentence has only one.