default value of drop down list

Hi,

Hope someone can help me, I've been doing a drop down list but here comes a question/requirement that I need to do a drop down list with a default value displayed.

example:
I have values 700, 500 and 355 in my drop down list. How can I set the value into 700 when the user is not selecting yet? or another scenario is after selection and the user deleted the previous selection then I need to show the default value still back to 700, is that possible?

thanks