Positive feedback loops amplify the initial response that triggered them, so without a self-limiting end result, they would be "explosive."
Negative feedback loops control a single variable, while positive feedback loops can control more than one variable. Thus, negative feedback loops are simpler and use less energy.
Negative feedback loops act to negate the response that triggers them, which helps to keep important variables like body temperature or blood sugar levels within acceptable ranges.
both A and B
both A and C