Answer:
Cathy went to the store
Step-by-step explanation:
The main clause of a sentence, also called independent clause, is that which has a subject and a verb and expresses a complete thought on its own, and therefore, can also stand as a full sentence.
In the sentence provided, there are two clauses: "Cathy went to the store" and "because we were out of milk". "Because we were out of milk" does not express a complete thought, therefore it is not the main clause. "Cathy went to the store," on the other hand, has a full meaning on its own and expresses a complete thought, thus, it is the main clause.