You are creating a SQL Server 2008 Reporting Services (SSRS) solution for a company that has offices in different countries. The company has a data server for each country. Sales data for each country is persisted in the respective data server for the country. Report developers have only Read access to all data servers. All data servers have the same schema for the database.
You design an SSRS solution to view sales data.
You need to ensure that users are able to easily switch between sales data for different countries.
What should you do?

A.
Implement a single shared data source.

B.
Implement multiple shared data sources.

C.
Implement an embedded data source that has a static connection string.

D.
Implement an embedded data source that has an expression-based connection string.

Explanation:
Tip: "easily switch between" = "expression-based connection string"

http://msdn.microsoft.com/en-us/library/ms156450.aspx
Expression-based connection strings are evaluated at run time. For example, you can specify the data source as a parameter, include the parameter reference in the connection string, and allow the user to choose a data source for the report. For example, suppose a multinational firm has data servers in several countries. With an expression-based connection string, a user who is running a sales report can select a data source for a particular country before running the report.

Design the report using a static connection string. A static connection string refers to a connection string that is not set through an expression (for example, when you follow the steps for creating a report-specific or shared data source, you are defining a static connection string). Using a static connection string allows you to connect to the data source in Report Designer so that you can get the query results you need to create the report.
When defining the data source connection, do not use a shared data source. You cannot use a data source expression in a shared data source. You must define an embedded data source for the report.
Specify credentials separately from the connection string. You can use stored credentials, prompted credentials, or integrated security.

Add a report parameter to specify a data source. For parameter values, you can either provide a static list of available values (in this case, the available values should be data sources you can use with the report) or define a query that retrieves a list of data sources at run time.

Be sure that the list of data sources shares the same database schema. All report design begins with schema information. If there is a mismatch between the schema used to define the report and the actual schema used by the report at run time, the report might not run.

Before publishing the report, replace the static connection string with an expression. Wait until you are finished designing the report before you replace the static connection string with an expression. Once you use an expression, you cannot execute the query in Report Designer. Furthermore, the field list in the Report Data pane and the Parameters list will not update automatically.