Participation is optional.

He chose to act on his own free will.

He is tackling his job with all his might and sincerity.

I am participating in the committee for the new project.

Mr. Ichii is my best friend.

I prefer tea over coffee.

This book has missing pages, so you can't read all the information.

This product has underfill, which reduces its quality.

He suffered from oxygen deficiency while climbing the mountain.

There is a serious lack in that contract.