Here's why:
* Subject: The subject is the person or thing that performs the action of the verb. In this sentence, the verb is implied (we can assume it's something like "is" or "sits").
* Complete Subject: The complete subject includes the subject and any words that modify it (like adjectives, adverbs, or prepositional phrases).
In this case, "a dog" is the subject, and "in the house" modifies it, telling us where the dog is located. Therefore, the complete subject is "a dog in the house."