During exercise, (B) __________ muscles in the walls of the veins contract and cause the vein walls to stiffen so more blood flows to the heart and lungs.

During exercise, the muscles responsible for increasing blood flow in the veins are called skeletal muscles. These muscles are located in the walls of the veins and contract when we engage in physical activity. The contraction of skeletal muscles puts pressure on the veins, causing the vein walls to stiffen and push blood towards the heart and lungs for oxygenation.

To gather more information on this topic, you can do a quick online search using keywords such as "muscle contraction in veins during exercise" or "how muscles increase blood flow during physical activity." This will lead you to various sources, including scientific articles or educational resources that explain in further detail the process of muscle contraction and its effects on blood flow during exercise.