The human cardiovascular system is considered closed because
a. blood is continuously circulated in the body.
b. blood provides nutrients to organs.
c. blood collects oxygen from the lungs
d. there are no holes for blood to enter or leave the system

d. there are no holes for blood to enter or leave the system

The cardiovascular system is also known as the circulatory system.

It is made up of the heart, the blood vessels namely arteries, veins and capillaries, and approximately five liters of blood.

The circulatory system's major functions are to transport  oxygen, hormones, and nutrients to every cell in the body, and to tale away cellular waste products from the cells such as carbon dioxide  and bring them to excretory organs  where they are expelled out of the body.

answer:

convection currents in the magma drive plate tectonics. heat generated from the radioactive decay of elements deep in the interior of the earth creates magma (molten rock) in the aesthenosphere. the aesthenosphere (70 ~ 250 km) is part of the mantle, the middle sphere of the earth that extends to 2900 km.