Explanation
Alibaba Cloud Express Connect is a service that enables high-bandwidth, reliable, secure, and private connections between different networks, such as VPC networks across regions, Alibaba Cloud accounts, and on-premise data centers1. It supports different connection methods, such as physical connections, virtual border routers, and Express Cloud Connect1.
Scenario A: Intranet communication between VPCs and Smart Access Gateway in customers different branch offices. This scenario can be achieved by using Express Cloud Connect, which is based on the hardware capacities of Smart Access Gateway and provides SD-WAN capabilities1. Express Cloud Connect allows you to connect your branch offices to Alibaba Cloud through a dedicated partner backbone network, and access VPC networks in all regions1.
Scenario B: Intranet communication between a VPC and servers in an external IDC. This scenario can be achieved by using physical connections, which are dedicated network connections between on-premise data centers and VPC networks1. You can lease a line from your ISP or work with an Alibaba Cloud partner to establish a physical connection to Alibaba Cloud1. This way, you can access VPC networks in all regions with high bandwidth and low latency1.
Scenario C: Intranet communication between two VPCs in different accounts and different CIDR Blocks. This scenario can be achieved by using peering connections, which are logical connections that enable communication between VPC networks2. Peering connections support cross-region and cross-account scenarios, and allow you to connect VPC networks with different CIDR blocks2. However, peering connections are not part of Alibaba Cloud Express Connect, but a separate service called Cloud Enterprise Network2.
Scenario D: Intranet communication between two VPCs under the same account in the same region. This scenario can also be achieved by using peering connections, which are logical connections that enable communication between VPC networks2. Peering connections support intra-region and same-account scenarios, and allow you to connect VPC networks with different CIDR blocks2. However, peering connections are not part of Alibaba Cloud Express Connect, but a separate service called Cloud Enterprise Network2. References: 1: Express Connect - Alibaba Cloud 2: Introduction to Cloud Enterprise Network - Alibaba Cloud Document Center

Explanation
A read-only RDS instance is a replica of the primary RDS instance that can handle read requests and increase the read capability of the database system. A read-only RDS instance inherits the network type (intranet or internet) of the primary RDS instance and cannot switch between them. Therefore, option D is the correct answer. References: Create a read-only ApsaraDB RDS for MySQL instance, Instance types for read-only ApsaraDB RDS instances

Explanation
Alibaba Cloud provides comprehensive security protection covering networks, hosts, applications, databases, and content for on-cloud clients, but users still need to pay attention to security protection issues. Alibaba Cloud and its customers are jointly responsible for the security of customers' applications built on Alibaba Cloud. With security responsibilities shared between Alibaba Cloud and its customers, Alibaba Cloud provides a secure infrastructure to decrease the enterprise security burden of customers. As such, customers can configure and use cloud products in a secure manner, thus relieving much of the underlying security burdens while allowing customers to focus more on their core business needs. However, customers still need to follow the best practices and guidelines for cloud security, such as using strong passwords, enabling multi-factor authentication, encrypting sensitive data, applying security patches, etc. Customers also need to monitor and manage the security risks and incidents of their cloud environment using Alibaba Cloud's security products and services, such as Security Center, ActionTrail, Cloud Firewall, etc. References: Cloud Security- Alibaba Cloud, Security & Compliance Center, Shared Responsibility