A Solutions Architect is designing an online shopping application running in a VPC on EC2 instances behind an ELB Application Load Balancer. The instances run in an Auto Scaling group across multiple Availability Zones. The application tier must read and write data to a customer managed database cluster. There should be no access to the database from the Internet, but the cluster must be able to obtain software patches from the Internet.
Which VPC design meets these requirements?
A. Public subnets for both the application tier and the database cluster
B. Public subnets for the application tier, and private subnets for the database cluster
C. Public subnets for the application tier and NAT Gateway, and private subnets for the database cluster
D. Public subnets for the application tier, and private subnets for the database cluster and NAT Gateway