AOS about I-864 and I-864A ?

[eba]

I'm a USC and am sponsoring my wife and I am filling out the i-864.
I've been a student and my income is extremely low and my mom is our cosponsor. We live at my mom's house now, we are a household of 3. Which form would my mom fill out, I-864 or I-864A? I am confused...
as I was filling out my i-864, my mom qualifies under the definition as a "household member" from my household (don't forget it's my mom's house but I am refering to it as "my household" from my perspective as I fill out my i-864 form) and it seems to me that she would fill out the i-864A, but the i-864 seems feasible, too, because she could check box D in part 2 as a joint sponsor... Any help would be greatly appreciated!

[eba]

onbody know ?

The conundrum lies in the fact that my mom is under the same roof as us and is also related to me by birth, of course, which meets the I-864A's definition of a "household member," and it seems to me, then, that I should fill in that form, but on the other hand, it seems equally correct if she just filled out an i-864, as you suggest, and checked box D in section 2 that says "I am a joint sponsor... etc."
I guess my question is: what is the test to use to determine which of these forms to use in a situation like this?

Jonah&****la