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.