If Bangladesh proposes a merger with India, what should be India's policy and response?

If Bangladesh proposes a merger with India, what should be India's policy and response?
Personally, I don't want Bangladesh to become part of India. Bangladesh is going to sink due to global warming. Bangladesh is situated on the crucial Ganges Delta which is at high risk of sinking due to global warming. Bangladesh is a flat country; the average elevation height of Bangladesh from sea level is a mere 15 m. Even a rise of 1-meter sea level can affect millions of Bangladeshi people. Some parts of Bangladesh already have started to witness the effects of global warming. The only choice for Bangladeshis is to immigrate to India. There would be an economic burden on India if 18 crores Bangladeshis decided to migrate to India. The best policy for India is to let Bangladeshis prosper. If they become prosperous, they can immigrate to other countries also and there would be less economic pressure on India. Merging Bangladesh with India is useless since Bangladesh is going to sink step by step no matter how hard you try. It is predicted that 1/4th of Bangladesh would be underwater by the end of this century.