Selecting Optionbutton using VBA

I have two option buttons on each of five spreadsheets in my Excel workbook.

Selecting the first option button enables an x to be written to a cell when that cell is clicked with the mouse.

Selecting the second option button disables the writing of the x when the cell is clicked with the mouse.

I have these working quite well.

My problem is that I want the second option button selected (disable writing of x) when I enter the sheet. I'm pretty certain that I need the code in Sub Worksheet_Activate() but I haven't been able to find the VBA statement to set the second option button on.

Also, unlike having the option buttons on a form, when I select the option button on the spreadsheet in Design mode I can see no property sheet indicating the name, value, etc of the option button.