Alkenes can be converted to cis-diols using $$\text{OsO}_4$$ and $$\text{NaHCO}_3$$ and to trans-diols via the epoxide followed by acid hydrolysis. 

Compound C given below is formed from :

i) alkene A by treatment with $$\text{OsO}_4$$ and $$\text{NaHCO}_3$$, 

ii) the epoxide of alkene B followed by acid hydrolysis.

The structures of A and B are

A

B

C

D