Geraint always cycles to work, leaving at $$8$$am every morning. When he averages $$15{\text{km}}/{\text{h}}\textbackslash;$$,he arrives $$10$$ minutes late. However, when he averages $$30{\text{km}}/{\text{h}}\textbackslash;$$, he arrives $$10$$ minutes early. What speed should he average to arrive on time? 
选项为:A:$$20{\text{km}}/{\text{h}}\textbackslash;$$  B:$$21{\text{km}}/{\text{h}}\textbackslash;$$  C:$$22.5{\text{km}}/{\text{h}}\textbackslash;$$  D:$$24{\text{km}}/{\text{h}}\textbackslash;$$  E:$$25{\text{km}}/{\text{h}}\textbackslash;$$  请回答其中一个选项.