You are deploying an application on App Engine that needs to integrate with an on-premises database. For security purposes, your on-premises database must not be accessible through the public internet. What should you do? 

Answer options:

A. Deploy your application on App Engine standard environment and use App Engine firewall rules to limit access to the open on-premises database.
B. Deploy your application on App Engine standard environment and use Cloud VPN to limit access to the on-premises database.
C. Deploy your application on App Engine flexible environment and use App Engine firewall rules to limit access to the on-premises database.
D. Deploy your application on App Engine flexible environment and use Cloud VPN to limit access to the on-premises database.

D