QID:RTE039 - please explain answer

[pablo701]

This question asks about configuring a default route.

The correct answer is:
    router eigrp 190
    redistribute static
    ip route 0.0.0.0 0.0.0.0 null 0

The incorrect answer is:
  ip default-network 20.0.0.0

(1)
Why is the "ip default-network 20.0.0.0" answer incorrect?
(2)
In the correct answer, why the "null 0"?
The question states the link toward the internet is e 0/0. So I expected  e 0/0 to be present rather than null 0?

thanks,

pablo