(a) Factors responsible for the very high population density in North - East of USA are:
(i) Presence of great deposits of iron and coal
(ii) Location of many industries
(iii) Favourable climate conducive for human habitation
(iv)Availability of jobs which make population to increase
(iv) Heavy trading of manufactured goods within and outside the industrial zone of USA.
(b) Factors which favour the high population density in the Indus and Ganges deltas
(i) Abundance of rainfall which ensures smooth agricultural activities
(ii) The soils are very fertile
(iii) Intensive agricultural activities attract people to the area (they grow 2-3 crops in a year)
(iv) Favourable climate conducive for intensive farming.