Leukocytes are produced/stored in thymus, spleen, and bone marrow
I think the second option is the best (by making the ramps height and length equal) because the first option will require more pushing, the third option will only make the ramp lighter which could cause it to not stay in place, the last option would make it harder to push the boxes up because the rubber will make it harder to slide the boxes up. I hope this helps:)
Answer:
The first line of defence (or outside defence system) includes physical and chemical barriers that are always ready and prepared to defend the body from infection. These include your skin, tears, mucus, cilia, stomach acid, urine flow, 'friendly' bacteria and white blood cells called neutrophils.
Explanation:
