One of the challenges of studying life from ancient times is that not all organisms form fossils. Which of the following explains why some organisms form fossils and others don't. (Select all that apply)

a. The tracks or burrows of an organism must be filled with sediments before they are disturbed.

b. The organism must first get stuck in rock.

c. The soft tissue of the dead organism must be eaten by scavengers or decomposers before the bones can be preserved.

d. The organism's body must be covered by sediment or another substance before its body decays.