You manage an ecommerce application that processes purchases from customers who can subsequently cancel or change those purchases. You discover that order volumes are highly variable and the backend order-processing system can only process one request at a time. You want to ensure seamless performance for customers regardless of usage volume. It is crucial that customers' order update requests are performed in the sequence in which they were generated. What should you do?

A. Send the purchase and change requests over WebSockets to the backend. B. Send the purchase and change requests as REST requests to the backend. C. Use a Pub/Sub subscriber in pull mode and use a data store to manage ordering. D. Use a Pub/Sub subscriber in push mode and use a data store to manage ordering.