Make Cell Mandatary To Select

Hi Team,
I have one example.
PFA Excel file.
Question?
In this example I have four form elements, all are drop-down list.
I want to make the first two select box as mandatory.
Like: 1. If no value selected for all select boxes - OK
2. If Only Mandatory select boxes selected - OK
3. If non-mandatory select boxes selected and mandatory select box values are blank - Error

Please provide the solution.
Thank!
Mandar.