Final answer:
Milk chocolate becomes toxic to dogs at the rate of 1 ounce per pound of body weight. It's vital to use pounds and ounces to evaluate and prevent the risk of chocolate toxicity in dogs, especially given their relative blood volume.
Step-by-step explanation:
The correct rule of thumb for the rate at which milk chocolate becomes toxic to dogs is option A, 1 ounce per pound of body weight. Chocolate contains theobromine and caffeine, which are toxic to dogs because they cannot metabolize these compounds as efficiently as humans. The toxicity level depends on the type of chocolate and the dog's body weight. Since an ounce is a suitable unit to measure the amount of chocolate and the pound is an appropriate unit for a dog's body weight, we can estimate the potential for toxicity using the ratio provided.
Considering that large dogs can weigh upwards of 100 lbs and the average volume of blood in a dog is about 85 ml of blood per kg of body weight, it's easier to grasp why even a small amount of chocolate, specifically in ounces, could have a significant impact on a dog's health.