In mathematics, equivalence classes are a very important concept.

It's very easy to walk on this flat road.

The initial response when an earthquake occurs is very important.

This statue is made of copper.

He found an old copper coin.

Mou is a common surname in China.

His foot got entangled in the rope.

He blushed, feeling a sense of shame.

This book is in 10 volumes.

He always follows the teaching of 'Never invite suspicions needlessly,' and avoids actions that may cause suspicion.