What is the human body's first line of defense against invading pathogens such as bacteria and viruses?

Aphysical change is one that you can put in and remove like a ham sandwich. the stuff you put in there can be changem its not permanet. a chemical change is on that can't be change thats permanet. like when you make a cake and bake it. you can't get those ingredients back.

Ithink it’s carbon dioxide

Ithink the answer is d.