Macro needed to add User Name and Date/Time Stamp in Tracking Sheet

I am trying to create a tracking sheet that will be used by several departments to track production of large documents. Since the activities may not happen in a strict order, the tasks are listed in a drop down list. When the user selects the activity in column A from a drop down list, I would like their user name to appear in column C and the date and time to appear in column D. I have attached a sample worksheet.

Thank you for any help possible.
-Evelyn