Prevent Postback in bootstrap Modal while data population.

Question

I'm trying to populate the dropdownlist from database in bootstrap modal, the issue is this, i want to populate other dropdownlist on the bases of first, like

Dropdownlist Provence is selected and the other City Dropdownlist will be populated on the bases of Provence Dropdownlist. but when provence is populated my modal gets closed by postback.

I'm using selected index changed of dropdownlist.

any guide/help will be heartly appreciated.