Does anyone know this one please help me stuck on it

Select all the true statements about the cosmic microwave background radiation

Adigestive system that is a series of tubes beginning at the mouth and ending at the anus is a digestive system.

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