वैश्विक स्तर पर 'प्रतिभा पलायन' (ब्रेन ड्रेन) मुख्य रूप से निम्नलिखित में से किस प्रवासन प्रतिरूप के माध्यम से होता है?

Snow houses (igloos) are typically found in regions:

For historical indigenous populations who lived in rugged terrains like the Andes and the Himalayas, mountain journeys were primarily:

Which of the following reasons is primarily responsible for the phenomenon of 'brain drain'?