I'm working my way through a practice exam for an AWS certification. One of the questions is as follows:
The web tier for an a pplication is running on 6 EC2 instances spread across 2 AZs behind a classic ELB. The data tier is a MySQL database running on an EC2 instance. What changes will increase the availability of the application? (select TWO) A: Turn on CloudTrail in the AWs account B: Migrate the MySQL database to a Multi-AZ RDS MySQL database instance C: Turn on cross-zone load balancing on the ELB D: Launch the web tier EC2 instances in an Auto Scaling Group E: Increase the instance size of the web tier EC2 instance
Correct answers are B and D. My question is, why is C NOT a correct answer? The instructor (an Amazon employee) glosses over C, explaining that "enabling cross-zone load balancing would have little to no effect on availability." But the way I'm looking at it, if the ELB can't send traffic to both AZ's, then we're effectively making our 6-instance system into a 3-instance system (assuming there are 3 in each AZ). And a single AZ system is never the considered a highly available architecture, since if that one AZ fails, your whole system is unavailable.