Sami is the Chief Executive Officer $($CEO$)$ of a publicly$-$held corporation. The corporation has recently expanded into new geographic regions. So Sami wanted to quickly hire a group of regional managers in three different continents. If you were Sami, what would be the most effective external recruiting method in this case?

a$.$ Headhunters$/$ Executive search firms

b$.$ On$-$Demand Recruiting Services $-$ ODRS

c$.$ Newspaper advertising

d$.$ College recruiting