what is it called when all of the organisms of a speicies completely die out. A= Mutations B= Adaptation C= Fossilization D= Extinction​

I’m doing some practice exam questions for biology, and i’m not sure how to do these 3 parts to the question.

An organ system consists of a group of organs that performs specific functions necessary for the survival of an organism. humans have eleven different organ systems: integumentary, skeletal, muscular, nervous, endocrine, circulatory, lymphatic, respiratory, digestive, urinary, and reproductive. discuss the function of neurons, the brain and the spinal cord within the human nervous system while explaining the interrelationship of each organ.

What element is able to combine with itself and hydrogen to form large molecules ?