Originating in Tibet, the Brahmaputra, Irrawaddy and Mekong rivers flow through narrow and parallel mountain ranges in their upper reaches. In these rivers, the Brahmaputra makes a U-turn in its flow just before entering India. Why does this make a U-turn?

• A. Reasons for the rise of folded Himalayan ranges
• B. Geologically young due to axial damping of Himalayas
• C. Due to geotectonic disturbances in the third ephemeral folded mountain ranges
• D. In this context both the reasons 'a' and 'b' above are reasonable.