A developer is trying to retrieve data over a REST API. The API server responds with an HTTP client error response code. After investigating the response, the developer realizes the response has a Retry-After header. What is the root cause of this error?

A. An appliance limited the rate of requests to the transport layer. B. The REST service is unreachable at the time of the REST request. C. Too many requests were sent to the REST service in a given amount of time. D. An appliance limited the rate of requests to the application layer.