Use the "Group" method, row by row, or a entire range

for (int i = upperRowNumber; i <= lowerRowNumber; i++)
{
cellA = savedWorksheet.get_Range(string.Format("{0}{1}",
ACellName, i), Missing.Value);
cellB = savedWorksheet.get_Range(string.Format("{0}{1}",
BCellName, i), Missing.Value);

if (cellA.Value2 == null)
{
if (cellB.Value2 != null)
(savedWorksheet.Rows[i, Missing.Value] as Range).
Group(Missing.Value, Missing.Value,
Missing.Value, Missing.Value);
}
}