Option C, 'Error handling in the subscriber code is not handling run-time errors properly,' suggests that the Cloud Functions might not be handling runtime errors correctly, hence no errors are logged in Cloud Logging. This indicates the errors are silently failing, leading to unprocessed messages. Option E, 'The subscriber code does not acknowledge the messages that it pulls,' means if the message acknowledgments are not happening, Cloud Pub/Sub will redeliver those messages, causing message accumulation. This results in an elevated message processing rate as the same messages continuously get reprocessed. Both these issues are likely causes for the observed behavior.