To be an index fossil, a fossil must
a. be present in rocks that are scatte.
over a large geographic area
b. contain remains of an organisms that
lived for a long period of geologic time
c. occur in small numbers within the rock
layers
d. have features that clearly distinguish it
from other fossils

a. They must be present in a large geographic area.

Explanation:

Index fossils can be used to determine the age of a rock strata with great precision, but they must be scattered over a large geographic area to be able to be used reliable for this type of measurement. Index fossils typically indicate a certain type of paleoenvironment belonging to a very specific prehistoric period and their formation coincides with the age of the rock layer.

when the chinese communist party took power in 1949, it began to build new di and levees to hold the yellow and yangtze rivers back.

i'm sure the answer you are looking for would be

a. an ice age

hope this contact me if this didn't or you need with more answers : )