You are a solutions architect working for a large travel company that is migrating its existing server estate to AWS. You have recommended that they use a custom Amazon VPC, and they have agreed to proceed. They will need a public subnet for their web servers and a private subnet in which to place their databases. They also require that the web servers and database servers be highly available and that there be a minimum of two web servers and two database servers each. How many subnets should you have to maintain high availability