3:they are usually microscopic
Answer:
A
Explanation:
Blow flow in the capillaries is slowest despite the large surface area. This is to allow for exchange of gases and nutrients at the cellular level.
Fats must be broken down into fatty acids before being converted into glucose. Lipids are known as the back up energy source. They are used to make enzymes, hormones, and antibodies.
In order for natural selection to take place we must have "variety in a population". This variety of traits is what allows natural selection to "pick" the most favorable traits.