The main advantage of using a library to find sources is because published works are often more reliable.
Answer:
I think it's C, the third option. Sorry if i'm wrong!
His example will show teachers
Many have heard it, but nobody has ever seen it. It will not speak back until spoken to. What is it? The answer to the riddle is an echo. Echo is the reflection of the sound that when it is released by the speaker, it arrives back to the speaker (now listener) but is delayed.
