what is the volume of cylinder? a).the circumference of the base is double the height. b) the surface area of the cylinder is 400 cubic meters. Is two statements alone can solve that question?

yes

In order to find the volume, you need both radius and height. From statement I you get the relation between radius and height but you don't get the radius or height or statement I is not sufficient. From statement two you only know the surface area but you can't find the volume.
If you use both statements then we can find the volume. So both statements are required to find the volume.