Dynamic stretches are part of an effective warm-up because they can
a) lubricate the muscles used in the activity
b) decrease cardiovascular difficulty in the activity
c) expand lung capacity for the activity
d) build endurance for the activity

The answer is B.

Dynamic stretching is a form of stretching used specifically before sports-related activity in order to maximize performance levels (i.e. range of motion and flexibility). Since it increases blood flow and sends this blood to your muscles, dynamic stretching will decrease cardiovascular (of the heart and blood vessels) difficulty in the activity.