Which organ is a part of circulatory system?
a. brain
b. heart
c. lungs
d. stomach

The brain is part of the neurological/nervous system, the lungs are part of the pulmonary system, and the stomach is the key component of the digestive system. This leaves the answe "B", the heart, as the answer because it is the pump in every mammalian body that drives blood flow leaving the heart through arteries and returning deoxygenated through the veins.

B is the answer   4 that        :)                           

Is there a diagram/picture you can provide? it seems this question goes along with a

All the elements are arranged by melting points on the periodic table. in row they go from metals to gasses. in the row from sodium to argon, sodium is more reactive than the one on its right, magnesium.