Based on the context in paragraph 4, fossil refers to . a. how igneous rocks were formed over time
b. a trace or print, or the remains of a plant or animal of a past age preserved in Earth or rock
c. layers of sediment building up over time and pressing down, forming rock

4. Sedimentary rock is the second category. These rocks cover most of Earth’s land. Sedimentary rocks are made of pieces of other rocks and shells. Erosion causes rocks to slowly wear away. Erosion can be caused by rain, wind, freezing, and plant roots. Sometimes small pieces of rock are made because of erosion. The small pieces of rock are called sediment. Rivers take sediment to oceans and lakes. The river drops the sediments as it slows down. The sediments settle to the bottom in layers. More and more layers of sediment are dropped over millions of years. The weight from the top layers turns the bottom layers of sediments into rock. This makes some sedimentary rocks look layered. Fossils are often found in sedimentary rocks.