A geologist is trying to determine what time period a layer of rock was formed. Fossil Experiment steps:

1. She identifies four active dig site locations in the southwest region of the United States.
2. She gathers seven different samples of the rock layer from each of the active dig sites.
3. She transfer the rock samples to a laboratory.
4. In the lab, she tests on sample from each site using radioactive dating to determine the rock formed 240 million years ago.

Which statement describes a weakness of this experiment?
a. She gathers seven rock samples from each site
b. She uses radioactive dating on only one rock sample from each site
c. She obtains the samples from active dig sites
d. She limits herself to only four locations to obtain samples