How to get data from SqlDataReader to Array of arrays or Array of strings

I'm trying to populate an array of arrays from a SqlDataReader, but in the end all rows of the array are filled with the same array of numbers.

Here's my T-SQL code:

SELECT 
    SUM(memberAttendants),
    Groups.Name,
    DATEPART(wk,convert(datetime,convert(varchar,StatisticsDate),104))
FROM 
    Stat.dbo.GroupStatistics 
INNER JOIN 
    Stat.dbo.Groups ON GroupStatistics.GroupID = Groups.GroupID 
WHERE 
    (Groups.OrganizationID = '4960') 
GROUP BY 
    Groups.Name, convert(datetime, convert(varchar, StatisticsDate), 104)
ORDER BY 
    Groups.Name, convert(datetime, convert(varchar, StatisticsDate), 104)

C# code:

SqlDataReader myReader1 = cmd1.ExecuteReader();

if (myReader1.HasRows)
{
    int i = 0, j = 0;
    string[] Name;
    Name = new string[100];
    int[][] MainArray = new int[100][];

    int[] ActivityArray;
    ActivityArray = new int[52];
    int[] WeekOfActivity;
    WeekOfActivity = new int[52];
    bool myFlag = false;

    for (int myCounter = 0; myCounter <= myReader1.FieldCount; myCounter++)
    {
        Aktivitet[myCounter] = new int[myCounter];
    }

    while (myReader1.Read())
    {
      if (!myReader1.IsDBNull(0) && !myReader1.IsDBNull(1))
      {
        if (i == 0 || myReader1.GetString(1) == Name[i-1]  || myFlag == true)
        {
           if (myFlag == true && myReader1.GetString(1) != Name[i-1])
           {
              i = 0;
              MainArray[j] = ActivityArray;
           }
           myFlag = false;
           ActivityArray[i] = myReader1.GetInt32(0);
           Name[i] = myReader1.GetString(1);
           i++;
       }
       else
       {
           MainArray[j] = ActivityArray;
           j++;
           i = 0;
           Name[i] = myReader1.GetString(1);
           ActivityArray[i] = myReader1.GetInt32(0);
           i++;
           myFlag = true;
       }
   }
}

MainArray[j] = ActivityArray;

How can I fill the MainArray without the last ActivityArray overwrite all the rows in MainArray?

Tnx