Please help! convert from surface model to solid model

[ohyata]

Hi,

I normally get "closed surface" iges file or step file. (see attached)
Both are same model, but different format.
I just don't know which is better format.

I'd like to convert closed-surface model to solid model,
Would you let me know how to?

and after converting, how can I check the model type (surface? or solid?)

I am using AutoCAD 2013 student version.

Thank you!