The experimental procedures used in Experiment 1 and 2 differed in that in Experiment 1:
A. incubation time was held constant, while drug concentration was varied.
B. incubation time was varied, while drug concentration was held constant.
C. incubation time and drug concentration were both held constant.
D. incubation time and drug concentration were both varied.

Experiment 1

Equal numbers of penicillin-resistant bacteria were put into flasks containing 10.0 milliliters of a nutrient medium. The flasks were incubated for 1 hour at 37^◦C with different concentrations of the 5 drugs shown in Table 1. A control consisted of bacteria incubated in the medium without any drugs. The bacteria were washed to remove residual drug traces and grown on nutrient agar plates for 7 days. During this time, the bacteria reproduced, forming colonies, which were then counted at the end of the seventh day. Plates with more colonies were assumed to have more live bacteria at the end of the 1-hour incubation period. Table 1 shows the number of colonies counted. The drug-free control showed 50 colonies at the end of 7 days.

Experiment 2
Bacteria were handled as described in Experiment 1 with two exceptions: all drugs were tested at the same concentration and the incubation time of each culture was varied. Table 2 shows the number of colonies counted for Experiment 2.

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Drug R S T U V Table 1 Drug concentration 10 15 Number of colonies: 26 5 41 42 45 47 50 29 35 38 40 9 12 20 21 22 Notes: *mM is micromolar (mm) 25 0 2 5 6 7 Numbers of colonies are averages for 5 replicates (identical samples).