A news feed web service has the following code running on Google App Engine. During peak load, users report that they can see news articles they already viewed. What is the most likely cause of this problem? 

Answer options:

A. The session variable is local to just a single instance
B. The session variable is being overwritten in Cloud Datastore
C. The URL of the API needs to be modified to prevent caching
D. The HTTP Expires header needs to be set to -1 stop caching

A