This question requires you to realise that the results are presented on a log scale. The greatest decline occurs on day 4 even though the line is not as steep because the difference between 10<sup>7</sup> and 10<sup>6</sup> is 10 times greater than the difference between 10<sup>6</sup> and 10<sup>5</sup>.

The reduction in size of the normal bacterial population is much closer to 100 000 000 than to 1 000 000