Correct answer choice is:
A.) First line of defense
Step-by-step explanation:
The immune system can be divided into three basic lines of defense against pathogenic infection. The first line of defense (or exterior defense mode) involves physical and chemical restraints that are perpetually willing and equipped to protect the body from germs. These contain your skin, cracks, excretion, cilia, belly acid, urine flow, 'friendly' bacteria and white blood cells known as neutrophils.