What is the most likely cause of these duplicate messages?

You work for a car manufacturer and have set up a data pipeline using Google Cloud Pub/Sub to capture anomalous sensor events. You are using a push subscription in Cloud Pub/Sub that calls a custom HTTPS endpoint that you have created to take action of these anomalous events as they occur. Your custom HTTPS endpoint keeps getting an inordinate amount of duplicate messages.

What is the most likely cause of these duplicate messages?
A . The message body for the sensor event is too large.
B . Your custom endpoint has an out-of-date SSL certificate.
C . The Cloud Pub/Sub topic has too many messages published to it.
D . Your custom endpoint is not acknowledging messages within the acknowledgement deadline.

Answer: D