TOEFL Listening: ETS-TOEFL听力机经 - 7H96Z4FZ9XO45WD4P$

Why does the professor mention small pieces of bluestone found at Stonehenge? A. To provide evidence that bluestone was believed to have healing properties. B. To stress that bluestone is not as durable as local stone. C. To help explain bow builders selected the particular stones used for Stonehenge. D. To express frustration with the way some researchers gather evidence at Stonehenge.