After send all select fields appear blank as default

In a mail, I noticed that a select box has been sent blank. but this should not be possible, because all select fields have fixed values.
Now I realized that after sending, all checkboxes in the browser appear blank. Now, if someone wants to make a second booking but he overlooks likely that he needs to choose this new and so it happens that empty values ​​are transmitted.

How can I after sending prevent the select fields to be displayed as blank?
Thank you!

http://wordpress.org/extend/plugins/contact-form-7/