It can be inferred from the passage that a bacterium would increase the likelihood of its moving away from an area where the concentration of a harmful substance is high if it did which one of the following? A. Increased the speed at which it swam immediately after undergoing the random changes in direction that result from tumbling. B. Detected the concentration gradient of an attractant toward which it could begin to swim. C. Relied on the simultaneous measurement of the concentration of the substance in front and back of its body, rather than on the comparison of the concentration at different points in time. D. Exhibited a complete cessation of tumbling when it detected increases in the concentration of substance. E. Exhibited an increased tendency to tumble as it encountered increasing concentrations of the substance, and suppressed tumbling as it detected decreases in the concentration of the substance.