Context: Elena 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 Elena investigate with an experiment?
A. Do small rocks or large rocks skip more times when thrown across the river?
B. Do rocks skip more times when thrown across a river or across a pond?
C. Do round rocks or flat rocks skip more times when thrown across the river?
From the given choices, pick the correct answer and respond by indicating the letter of the correct option.

A