everyone / pick up / one piece of trash / a day / about 6.6 billion pieces less.

• A. 
  If everyone picked up one piece of trash a day, there would be about 6.6 billion pieces less.
• B. 
  When everyone will pick up one piece of trash a day, about 6.6 billion pieces would be less.
• C. 
  Because everyone picks up one piece of trash a day, about 6.6 billion pieces is less.
• D. 
  Unless everyone picked up one piece of trash a day, there will be about 6.6 billion pieces less.