Six Strains of the Same Bacterium Are Being Grown Together

Six strains of the same bacterium are being grown together with Penicillium mold.One of the bacterial strains has grown up to the edge of the mold colony, while the other five have not.Which of the following is the best explanation for this difference?

A) It is a random event; in another trial five of the six bacterial strains would be just as likely to grow to the edge of the mold colony.
B) One bacterial strain grows faster than the others.
C) The mold secretes a chemical to attract the bacteria to use for its food, but only one bacterial strain responds to it; the others are resistant.
D) One bacterial strain is resistant to the antibiotic produced by the mold, while the others are inhibited by it.
E) The plate is slanted, so one strain has been growing "downhill" and the others sideways or "uphill" toward the mold.