Alaboratory culture contains 150 bacteria. after twenty minutes, the culture contains 300 bacteria. after one hour, the culture contains 1200 bacteria. make a conjecture about the rate at which the bacteria increases

The answer is C..!! 
see.. at t=0 the culture had 150 bacteria.. 
at t=20 the culture had 300 bacteria. 
at t=60 the culture had 1200. 
so if u go by the second option... at t=40 the bacteria would be 600.. 
i am just telling you the logic..!! if u have to formally solve it.. use the concept of sequences and series... :)

The correct answer is: b