Meat from chickens contaminated with salmonella bacteria can cause serious food poisoning. Capsaicin, the chemical that gives chili peppers their hot flavor, has antibacterial properties. Chickens do not have taste receptors for capsaicin and will readily eat feed laced with capsaicin. When chickens were fed such feed and then exposed to salmonella bacteria, relatively few of them became contaminated with salmonella.

In deciding whether the feed would be useful in raising salmonella-free chicken for retail sale, it would be most helpful to determine which of the following?

(A) Whether feeding capsaicin to chickens affects the taste of their meat
(B) Whether eating capsaicin reduces the risk of salmonella poisoning for humans
(C) Whether chicken is more prone to salmonella contamination than other kinds of meat
(D) Whether appropriate cooking of chicken contaminated with salmonella can always prevent food poisoning
(E) Whether capsaicin can be obtained only from chili peppers

OA is A

I have a doubt in D. Please clarify it.
If i say, Appropriate cooking of chicken contaminated with salmonella do not always prevent food poisoning, then in that case it strengthens the argument by saying that "feed would be useful in raising salmonella-free chickens.

However, if say "appropriate cooking always prevent food poisoning", then doesn't it weaken the argument.

Please let me know your thoughts on this above