Explanation
According to Appian best practices, environment-specific constants should be used to define values that vary from one Appian environment to another. For example, if you want to generate an email body that contains the environment name, you can create a constant named ENVIRONMENT_NAME and assign it different values for DEV and TEST environments. This way, you can avoid hard-coding values in your expressions or rules, and simplify your deployment process by using automatic constant value mapping.

Explanation
The database object that should be used to create a virtual relation in a database to query the data is a view. A view is a searchable object that is defined by a query. It does not store any data but retrieves data from one or more tables at run time, creating a virtual table and returning the data in the defined format. It is created using joins, filters, aggregations, or other SQL operations. A view can be used to simplify complex queries, provide security, or enhance performance. Therefore, the correct answer is D.
References:
* Relational Database Guidance
* Views

Explanation
Two true statements are:
* It is not possible to undo changes from an import to all types of objects. Once a package is imported to an environment, it applies changes to the existing objects or creates new objects based on the package contents. There is no built-in mechanism to revert these changes or delete these objects automatically.
Therefore, it is important to test the package in a non-production environment before importing it to production, and to backup any objects that may be overwritten by the import.
* It is possible to import the same package multiple times. A package can be imported to an environment more than once, as long as it does not contain any conflicts or errors that would prevent the import.
However, importing the same package multiple times may not have any effect if there are no changes in the package contents or in the target environment since the last import. References: Prepare Deployment Packages, Application Deployment Guidelines