What Does the Term "Primary" in the Diagnosis of Primary

What does the term "primary" in the diagnosis of primary insomnia refer to?

A) the fact that two co-occurring disorders are not present
B) the fact that the sleep disorder is a major cause of other medical or psychiatric conditions that the individual is experiencing
C) the fact that the sleep disturbance is not due to another medical or psychiatric condition
D) the fact that the person has never been diagnosed with insomnia before