Task: Question Answering
Input (English): Problem: david works at a science lab that conducts experiments on bacteria . the population of the bacteria multiplies at a constant rate , and his job is to notate the population of a certain group of bacteria each hour . at 1 p . m . on a certain day , he noted that the population was 500 and then he left the lab . he returned in time to take a reading at 4 p . m . , by which point the population had grown to 13,500 . now he has to fill in the missing data for 2 p . m . and 3 p . m . what was the population at 3 p . m . ? 
Options: a ) 6500 , b ) 3500 , c ) 2500 , d ) 4500 , e ) 2000
Output (English): d