First value in drop down option should be disabled - Rails

First value in drop down option should be disabled - Rails

I have dropdown list which has the following options
---select an option---
option 1
option 2
option 3
option 4
I want to disable my 1st value ---Select an option--- in dropdown list. So
that it could not be selected as an option.
My code is here
<%= f.select :options, options_for_select([[" ---Select an option---",
""], "option 1", "option 2", "option 3", "option 4"]), {}, :style =>
"margin:20px; width:356px;" %>
How can I implement that??? Thanks.