7. the skeleton that protects the body's vital organs immune system

The immune system works to keep germs out of the body and destroy any that get in. The immune system is made up of a complex network of cells and organs thatprotect the body from infection. Lymph nodes are part of the immune system. They release lymphocytes, a certain type of white blood cell that fights infection.

"d.) carbon dioxide and hydrogen sulfide"

allowed for the prediction of the properties of missing elements.