What is the by product of the cell's use of oxygen for fuel?

The purity of ecstasy is always in question, why? (short answer)

Why is a lunch consisting of pizza, fruit, and salad a better choice than a lunch with the same number of calories from pizza alone? a)both lunches provide the same amount of energy but the lunch of pizza, salad, and fruit provides more nutrients. b)both lunches provide the same amount of nutrients but the lunch of pizza, salad, and fruit provides more energy. c)both lunches are healthful but the lunch of pizza, salad, and fruit provides more energy for metabolism. d)both lunches are healthful but the lunch of pizza, salad, and fruit is likely to have a higher fat content.

Why is sustainable agriculture so important?