Answer: Exponential growth means that bacteria is growing at a high rate. So bacteria that is growing exponentially is more effective then introducing new bacteria into the oil spill.
Explanation: When bacteria is introduced into a new medium(oil spill) or environnment it first needs to adapt at an initail phase of its growth called the lag phase, that is when bacteria is still adapting and genetically expressing molecules and enzymes that will assist in digesting that particular source of nutrient in this case, Oil. Therefore an already growing bacteria to clean up an oil spill will be more effective then introducing another bacteria that has to first adapt to its changed environmental condition before it begins cleaning up by digesting oils as its energy source.