题目
Using the EC2 API, you requested 40 m5.large On-Demand EC2 instances in a single Availability Zone. 20 instances were successfully created but the other 20 requests failed. What’s the solution for this and what is the root cause?
A. For new accounts, there is a soft limit of 20 EC2 instances per region. Submit an Amazon EC2 instance Request Form in order to lift this limit.
B. You can only create 20 instances per Availability Zone. Select a different Availability Zone and retry creating the instances again.
C. A certain Inbound Rule in your Network Access List is preventing you to create more than 20 instances. Remove this rule and the issue will be resolved.
D. The API credentials that you are using has a limit of only 20 requests per hour. Try submitting the request again after one hour.
注解
Amazon EC2 有每个区域20个实例的软限制，可以通过EC2实例的需求表单提交您的实例请求来轻松解决这个问题。
选项2是不正确的，因为实例的限制不是在A-Z的；
选项3是不正确的，网络访问列表是你的VPC安全方面的一个选择，它像防火墙一样，为的是控制子网的流入或流出。不会影响到创建新的实例中的。
选项4是不正确的，你的API证书是不会有问题的。
相关链接：
https://aws.amazon.com/ec2/faqs/#How_many_instances_can_I_run_in_Amazon_EC2
http://docs.aws.amazon.com/general/latest/gr/aws_service_limits.html
答案
A