Choose the best sentence - a, b, c or d made from the given cues and the sentences which is closest in meaning to the printed one.

the world/ energy resources/ take/ millions of years/ produce/ we/ quickly/ begin/ exhaust/ supplies//

• A. 
  The world's energy resources have taken many millions of years to produce, so we are quickly beginning to exhaust these supplies.
• B. 
  Although the world's energy resources have taken many millions of years to produce, we are quickly beginning to exhaust these supplies.
• C. 
  The world's energy resources have taken many millions of years to produce because we are quickly beginning to exhaust these supplies.
• D. 
  Since the world's energy resources have taken many millions of years to produce, we are quickly beginning to exhaust these supplies.