Plz help
A prehistoric clamshell is buried by sediment on the seafloor. Over time, the sediment turns to rock, and the clamshell dissolves away. This leaves behind a cavity in the rock that is shaped like the clamshell.

What kind of fossil is this clamshell-shaped cavity?
A.
a mold
B.
a carbonized fossil
C.
a petrified fossil
D.
a cast

The rock and fossil record shows that
A.
both gradual and sudden changes have occurred during Earth's history.

B.
all of the changes during Earth's history have been slow and gradual.

C.
the Earth has not changed very much over the course of its history.

D.
all of the changes during Earth's history have been very sudden.