Dynamic Dropdown Choice

My question is, I have installed contact form 7 and everything works perfectly. I need an option when a customer wants to select pre-define CITY, and after selecting city he should select pre-defined Street for example:

Name: TextBox
Surname: TextBox
e-mail: textbox
City: DropDown List
Street: DropDown list
Comment:

The two tabs City and Street should be somehow connected, for example, if I choose One town, the only adresse from that town should be open and available for choice, if I select another town then the streets will be available that town.

Thanks

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