which expression should be used?

You are a database developer and you have about two years experience in reating business Intelligence (BI) by using SQL Server2008.

Now you are employed in a company named NaproStar which uses SQL Server2008. You work as the technical support.
Now you receive an e-mail from your company CIO, in the e-mail, the CIO assigns a task to you.
You use SQL Server 2008 Reporting Services (SSRS) to create a solution.

Your solution includes a time dimension named DimDay.
And the DimDay dimension contains a hierarchy named Yr-Qtr-Mth.
There is a measure group named Measures01 which contains a measure named Number.

Now a calculated member has to be created, then it will be used to display the Number measure in the previous time period for the current level of the Yr-Qtr-Mth hierarchy.
In the following options, which expression should be used?

A.
([Measures01].[Number],OPENINGPERIOD( [DimDay].[Yr-Qtr-Mth].FirstSibling.Level))

B.
([Measures01].[Number],LASTPERIODS( 0,[DimDay].[Yr-Qtr-Mth].PrevMember))

C.
([Measures01].[Number],[DimDay].[Yr-Qtr-Mth].PrevMember)

D.
([Measures01].[Number],[DimDay].[Yr-Qtr-Mth].FirstSibling)

Explanation:
Functions for navigating Hierarchies
MDX provides functions such as PrevMember, Children, and Parent for navigating hierarchies.
PrevMember takes a member and returns a previous member in the hierarchy. For example, [Order Date].[Calendar].PrevMember returns the previous member with respect to the current
member of the Calendar hierarchy. So if the Calendar hierarchy is expanded to the Year level and the current cell is referenced by year 2004, PrevMember will return year 2003.

EXAM TIP
The PrevMember function is frequently used to defi ne the KPI Trend or Goal properties. for example, the Reseller Sales KPI uses the following expression to set the Goal property to 40 percent more than the Reseller Sales Amount for the previous date period: 1.40 * ([Date].[Calendar].PrevMember, [Measures].[Reseller Sales Amount])
Consequently, if the user browses the cube data by years, the PrevMember function will return the previous year for each year. If the user browses data by quarters, the PrevMember will return the previous quarter for each quarter, and so on.