Actually - the Challenge is to explain how top C first can crush intact, strong A into dust B and then how intact top C in contact with B becomes more B. US presidents think Arabs did it ...
The thing is, top C didn't crush intact, strong A all at once. Top C crushed the top floor of A, and then the next floor down, and then the next floor down after that, and so on and so on. Also, I wouldn't call the top 10-20 stories of a building "light". That's still quite a bit of mass coming down with quite a lot of force.
Hm, top C consisted of 10 floors and is considered
rigid (!) by terrorists and bottom A of 100 floors, which are
not rigid! So
rigid top C first crushed down the uppermost floor of A and then the next, and the next until all 100 floors of A were dust B. And finally dust B crushed up from below top C, suddenly
not rigid (!) into more dust.
http://www.civil.northwestern.edu/people/bazant/PDFs/Papers/466.pdf . Sorry I didn't think of that. Imagine dust crushing up a
rigid thing!!!
Challenge (topic) is of course to show it scientifically and not invent garbage like prof. Bazant.
I have actually done it. Dropping any mass C of any mass A keeping C in place to start with always results in C bouncing on A, i.e. A stops C from falling creating any dust B at all. A acts as a shock absorber!
That's why nobody has won my
Challenge!
BTW I consider Bazant a terrorist supporting terrorism. I informed FBI about it and ... FBI was sleeping as usual.