how could i fetch the match values regarding one drop down to another drop down

hi people, as a beginner,
 
in a web form, i am having two drop down controls, and in the first one control  i am having, state names like some thing, and in another i have to fetch the concurrent city names for the first control, like if i select city name as 'A' it must fetch the related city names for the 'A'
how could i do this. please help.
give me e beginner idea.
 
thanks in advance.
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