What rule prevents us from taking the most largest group when there are don't care combinations in K-Map?

[oslon]

This is the K-map problem:

This is my solution.

But it's wrong as teacher says you can't pick that quad when you're with don't cares. Why? I don't know. Should not I pick the largest group? What's the edge conditions for this? Any ideas?

 

[tumbleweed]

Logically I think you're right, but if you're trying to minimize the terms then you can discard the don't cares.

Here's the logic of the first one:

if (A=0 and B=0) or (A=1 and B=1) then
    out = 1
else
    out = 0

Here's the logic of your solution (replacing the 'dont cares' with a 1):

if (A=0 and B=0) or (C=1) or (A=1 and B=1) then
    out = 1
else
    out = 0

In your solution you end up with a 'C=1' term, which you really don't need