Farmer brown believes that increasingly large doses of antibiotics will produce increasingly large beef cattle. what is the correct match between experimental variables and activities in farmer brown's experiment?
a) dependent variable; all cattle receive the same hay, water, and living quarters.
b) control variable; different groups of cattle are given different doses of antibiotics.
c) independent variable; farmer brown tests only antibiotic x in even incremental dosages.
d) hypothesis; farmer brown observes that dosage with 500 mg of antibiotics produces the most weight gain.

The answer is B, control variable; Different groups of cattle are given different doses of antibiotics.

Explanation:

Control variable is a variable that is held constant in order to assess or clarify the relationship between other variables in an experimental design. Essentially, a control variable is what is kept the same throughout the experiment, and it is not of primary concern in the experimental outcome. Any change in a control variable in an experiment would invalidate the correlation of dependent variables (DV) to the independent variable (IV), thus skewing the results.

The concern of the experiment is the relationship between antibiotics and growth rate, hence the animals will be grouped in at least 3 treatment groups and each would be given different level of antibiotics, and a control group which won't be given any antibiotics. Though every other variable like feed, drink, temperature etc will be same for all groups including the control group in order to prevent bias.

At the end of the experiment, the control group which wasn't given any antibiotic will be compared with those groups which were given, hence the group which performs best would be determined.

substitution is the answer in this case