By definition, EF/EA=ED/EC=1/2. In triangles EAC and EFD there is a common included angle E. Also EF/ED=EA/EC, because EF/EA=ED/EC. Therefore the two triangles are similar (two sides and included angle) and ED/DF=EC/AC. ED=1/2EC=16.5. So 16.5/20=33/AC, therefore AC=40 (answer A).