Dr James Isolated Staphylococcus Aureus, a Type of Bacteria, from the Leg

Dr. James isolated Staphylococcus aureus, a type of bacteria, from the leg wound of a ten-year-old boy. He suspected these bacteria would grow better at body temperature than room temperature (72°F) , but thought that he should collect data to support his thinking. Dr. James introduced the same number of Staphylococcus bacteria into each of six test tubes containing the same type and amount of nutrient broth. Three test tubes were incubated at 98.6°F (Group 1) , while three test tubes (Group 2) sat at 72°F. After 24 hours, Dr. James compared the turbidity (indicative of growth) of all six tubes and rated each on a scale of 0 - 4. 0 indicates no turbidity (no growth) , while 4 indicates high turbidity (high growth) . The following data were collected:
$$\begin{array} { | c | c | c | } \hline \text { Test Tube } & \text { Temperature } ^ { \circ } \mathrm { F } & \text { Turbidity } \\\hline \text { Group 1: } & & \\\hline 1 & 98.6 & + 4 \\\hline 2 & 98.6 & + 4 \\\hline 3 & 98.6 & + 3 \\\hline \text { Group 2: } & & \\\hline 4 & 72 & 0 \\\hline 5 & 72 & + 1 \\\hline 6 & 72 & 0 \\\hline\end{array}$$
-The dependent (responding) variable in this experiment is

A) the temperature.
B) growth of bacteria,as indicated by the turbidity in the test tubes.
C) the time that the test tubes were allowed to sit.
D) amount of initial inoculum,or number of bacteria introduced into each test tube.