Ashark would not be a good index fossil because

a. salt water would degrade the fossil over time.
b. no fossil records exist for sharks.
c. it doesn't have structures that can be preserved in a fossil.
d. the species has survived for too long.

I have to disagree, water-dwelling creatures are often the most well preserved due to the salinity of the water and the thick sediments on the sea floor that bury/fossilize skeletons.

I believe it is D. the species had survived for too long.

Sharks have existed and relatively remained the same for such a long time that shark fossils would not be good indicators of a certain time period.

c.more precipitation,but only when emission levels.