Context: Riley likes to skip rocks at the river. She notices that some of the rocks she throws skip off the surface of the water many times, while others skip once and then sink. She wonders what factors affect how well rocks skip on the water. So, she decides to design an experiment. She has the following supplies available:
10 small flat rocks
10 large flat rocks
access to the river by her school
Using only these supplies, which question can Riley investigate with an experiment?
A. Do rocks skip more times when thrown across a river or across a pond?
B. Do round rocks or flat rocks skip more times when thrown across the river?
C. Do small rocks or large rocks skip more times when thrown across the river?
Answer with the option's letter from the given choices directly.

C