Option Buttons De-selection

I am working in Excel 2000 and created a chart with two
columns of option buttons and 20 rows. Column one is a
column for 'YES', and column 2 is a column for 'NO'. I was
always able to select from row to another the option of
yes or no until recently. Now everytime I select yes or no
in one row and go to the next row and select yes or no,
the prvious selection in the row before it becomes de-
selected.

I am hoping that maybe somebody can help me out.

Thank you,
June