How do you distinguish between GMOs and Non-GMOs using PCR?

You want to test the DNA of two vegetables to see which one has been genetically modified. You then perform PCR for both templates of DNA. If the DNA is from a GM crop, the you will see that the product is 500 bp long. If there is non-GMO DNA, then the product will be 200 bp long. Six reactions are carried out below:

What would the gel with the following samples/primers in each lane look like?
1.  Non-GMO food with plant primers
2.  Non-GMO food with GMO primers
3.  Test food with GMO primers
4.  Test food with plant primers
5.  GMO positive DNA with plant primers
6.  GMO positive DNA with GMO primers.

1) If both test foods are genetically modified, what size bands would you expect from each reaction?
2) Which of the above reactions will contain positive controls in the lanes? Which ones will contain negative controls?

My answer is that all non-GMOs will show no bands in the gel, while GMOs will show a banding pattern that is 500 bp in length. I do not know where to begin.