निम्नलिखित में से कौन सा एक अपेक्षाकृत स्थायी संदर्भ बिंदु (point of reference) है जिसकी किसी माने गए डेटम (datum) के सापेक्ष ऊंचाई ज्ञात होती है?

The 'Drain of Wealth' theory was propounded by which Indian nationalist leader?

Which event is considered the turning point in Mahatma Gandhi's leadership in the Indian independence movement?

Which African leader is known for his non-violent opposition to colonial rule?