What is the common reason behind Brunei,Bahrain and Kuwait having a high income per head?

or:What is the common reason behind Brunei,Bahrain and Kuwait having a high income per head?or:It's because of the oil extracts

or:What is the common reason behind Brunei,Bahrain and Kuwait having a high income per head?


or:It's because of the oil extracts

Tags:high,head,