You are to non-culturally detect the presence of E. coli in the soil from a cow pasture.?

You are to non-culturally detect the presence of E. coli in the soil from a cow pasture. You do this by extracting the total DNA from a sample of the soil and PCR amplifying with E. coli-specific primers that bind to a gene only found in E. coli that amplify a region of the DNA 167 bp long. Propose a postive sample that could be tested alongside your experimental sample.
A. Known Shigella DNA
B. Water
C.umber of Injections
D. Muscle DNA
E. Known E. coli DNA

is it just the known E.coli dna? its the positive control that i can see in this context otherwise i could be wrong.