Which expression in the Group properties dialog box should you use?

You design a report by using SQL Server 2008 Reporting Services (SSRS).
The report uses a Table data region and contains a dataset. The dataset has a column
named LastName.
You need to ensure that the report items are grouped by the first character in the LastName
column.
Which expression in the Group properties dialog box should you use?

A.
=First(Fields!LastName)

B.
=Fields!LastName.Value