Agraph of coal reserves in the united states versus time slopes downward. this indicates that coal production could have a. stayed the same b. decreased c. increased d. fluctuated i think it’s d

The correct answer is:  [B]:  "decreased" .

Note:

        In a graph of "coal reserves in the United States" vs. "time" ; 

→ the "time" is the "independent variable" that belongs on the "x-axis" .

→ The quantity or amount of "coal reserves in the United States" is the "dependent variable" that belongs on the "y-axis" . 

       If the graph "slopes downward" ;  this would indicate a "decrease" in the amount/quantity of "coal reserves" over "time" .  

→  As the value of "x" increases — "time" increases — and; as "x" increases, the value of "y" {the amount/quantity of "coal reserves"} decreases.  

→ This would be represented by a "downward sloping" graph;  

→  and, among the answer choices given, could indicate that coal production:

                     →  "decreased" ;    

→  which is:  Answer choice:  [B]:  "decreased" .