Use the results of the first dropdown for the second one

Hi pals,
I have two tables like below:
Table 1:
countryId       countryName
1                   USA
2                   England

Table 2
cityId            cityName        countryId
1                 London            2
2                  LA                   1
3                  Manchester     2
4                 Washington      1
I have two dropdowns, named countryDD and cityDD. The result of the first one must effect the second one. For example, if USA is chos ...