The first element of an arithmetic sequence is $$\frac{1}{3}$$ and the third element is $$\frac{1}{5}$$. What is the second element of this sequence? 
选项为:A:$$\frac{1}{4}$$  B:$$\frac{4}{15}$$  C:$$\frac{5}{24}$$  D:$$\frac{7}{30}$$  E:$$\frac{2}{9}$$  请回答其中一个选项.