How do bacteria in the guts of cows help the cows to digest the grass they eat?
A.
They perform photosynthesis to provide sugar for the cow.
B.
They break down the cellulose cell walls of the grass to release nutrients that the cow can use.
C.
They catch particles of food from the grass in the digestive tract.
D.
They reproduce in very large numbers so that the they are used as food.