Read the passage from President Barack Obama’s address on Syria. On August 21st, these basic rules were violated, along with our sense of common humanity. No one disputes that chemical weapons were used in Syria. The world saw thousands of videos, cell phone pictures, and social media accounts from the attack, and humanitarian organizations told stories of hospitals packed with people who had symptoms of poison gas.

What evidence does Obama mostly rely on in this excerpt?

a. experiments and expert opinions
b. analogies and testimonials
c. visual evidence and anecdotes
d. personal observations and facts