This year Gareth is seven times the age of his son Herbie. In six years\textquotesingle{} time Gareth will be four times as old as Herbie. What is the difference between Gareth\textquotesingle s and Herbie\textquotesingle s ages this year? 
选项为:A:$$16$$ years  B:$$20$$ years  C:$$24$$ years  D:$$36$$ years  E:$$42$$ years  请回答其中一个选项.