$$10$$ trees are planted at equal distance along a river. The distance between the $$1^{st}$$ and the $$3^{rd}$$ tree is $$20$$ metres. What is the distance between the $$7^{th}$$ and $$9^{th}$$ tree? 
选项为:A:$$10$$  B:$$20$$  C:$$50$$  D:$$70$$  E:$$90$$  请回答其中一个选项.