Answer:
Falling into the lake was the reason she was soaking wet.
Step-by-step explanation:
Simple subject - A word or phrase that a sentence is about.
Complete subject - A simple subject and all of the words that a describe or apply minor changes to it.
Simple subject: Falling into the lake was the reason she was soaking wet.
The sentence is discussing the reason why the girl was soaking wet, “the reason” is vague, but tells what the sentence is mainly about.
Complete subject: Falling into the lake was the reason she was soaking wet.
This adds to the simple subject and describes “the reason” a bit more by adding in a few extra words.