Answer:
The best answer is A:
A. Clams have very hard shells, whereas spiders have soft shells that break down more easily.
Step-by-step explanation:
Spiders are any of numerous predaceous arachnids of the order Araneae, most of which spin webs that serve as nests and as traps for prey.
Arachnids are any wingless, carnivorous arthropods of the class Arachnida, including spiders, scorpions, mites, ticks, and daddy-longlegs, having a body divided into two parts, the cephalothorax and the abdomen, and having eight appendages and no antennae.
The oldest fossils of spiders are 52 million years old.
Fossils are any remains, impression, or trace of a living thing of a former geologic age, as a skeleton, foot print, etc.
For a fossil to form, the remains of an organism must become mineralized through a process of permineralization or through replacement of organic material by mineral deposits.
Clams have hard shells made of calcite, a mineral form of calcium carbonate. These shells are resistant to chemical and biological breakdown and can become fossilized relatively easily.
In contrast, spiders have soft exoskeletons made of chitin and proteins. These are susceptible to decay and decomposition, making spider fossils rare and incomplete in the fossil record. Their fossilization likely requires exceptional circumstances of rapid burial and mineralization.
Options B, C and D are not supported as the primary reason for the disparity in the fossil records of clams and spiders. Though spiders are small and possibly less abundant, some small arthropods do still appear as fossils. And spiders have been around for over 300 million years, so they are not newly evolved.