How to make the bottom off an extruded surface flat?

Rhino Rhino for Windows
1

How can i make the bottom of this extruded surface flat?

71081193_678697059275639_6972612601687572480_n
71081193_678697059275639_6972612601687572480_n1333×750 372 KB
71495488_409918133042096_909235630146322432_n
71495488_409918133042096_909235630146322432_n1333×750 144 KB
Agnostic.3dm (3.3 MB)

2

you can extract the cylinder, run UntrimAll to get the original surface, then Join and then Cap to close the object.