Compound A (molecular formula C$_3$H$_8$O) is treated with acidified potassium dichromate to form a product B (molecular formula C$_3$H$_6$O). B forms a shining silver mirror on warming with ammoniacal silver nitrate. B when treated with an aqueous solution of H$_2$NCONHNH$_2 \cdot$ HCl and sodium acetate gives a product C. Identify the structure of C.