Final answer:
To combine the two simple sentences 'I pet the cat' and 'The cat purred', you can use the coordinating conjunction 'and' to make a compound sentence: 'I pet the cat, and the cat purred'.
Step-by-step explanation:
A compound sentence contains two or more main clauses that are equally important to the meaning of the sentence. You can create compound sentences by using coordinating conjunctions like 'and' to join the main clauses.
So, to combine the two simple sentences 'I pet the cat' and 'The cat purred', you can use the coordinating conjunction 'and' to make a compound sentence: 'I pet the cat, and the cat purred'.