James created an XHTML table to show his schedule for each week of the month. He organized the table in a standard calendar format, so that each day from Sunday to Saturday is the header for a vertical column, and each week of days is displayed horizontally as a row. Each scheduled activity appears in the table cell appropriate for the day it occurs. James wants to merge two table cells into one cell to show the IT Business Conference that he will attend from Tuesday to Wednesday in the third week. Which tag should James use for this merged cell?