The first line of defense involves which structure(s)?
O T-cells
O skin
O blood
O B-cells

How many major plates cover the earth

What are the two causes of density in deep current waters? a. salinity (how much salt) of the water and high temperaturesb. salinity (how much salt) of the water and low temperatures c. oxygen content of the water and high temperatures. d. oxygen content of the water and low temperatures

There are fewer than 100 naturally occurring elements in the universe, but there are millions of different unified substances. explain how this can be true.