In the Adventure Park, 30 children took part in two of the adventures. 15 of them participated in the "moving bridge" contest, and 20 of them went down the zip-wire. How many of the children took part in both adventures? 
选项为:A:$$25$$  B:$$15$$  C:$$30$$  D:$$10$$  E:$$5$$  请回答其中一个选项.