Which of the following statements is true?

A. More than 50 chemicals in tobacco are known to cause cancer.
B. Eating disorders are more common in boys than in girls.
C. Liver scarring is a serious health issue caused by bulimia.
D. Smoking causes lung cancer, but it does not harm other body organs.

They carry out the exchange of gases as we breathe ( oxygen and carbon dioxide).

It is a mixture. simply put, a compound must be pure, that is, of only one chemical structure and composition. a mixture is made of different chemical substances (i.e. elements, molecules, compounds) that are not chemically bonded to each other, but instead physically grouped together, as in the case of gasoline (a homogeneous solution).

Combustion is the act or process of burning. therefore, i believe the answer is  a: burning wood.