Question

alice is performing an experiment to determine which mouthwash kills the most bacteria. she is testing three different mouthwashes. she uses four pieces of filter paper. each piece of filter paper was 3 cm in diameter. three of the pieces were soaked in each mouthwash for 3 minutes each. the fourth piece was soaked in plain water for 3 minutes as well. each piece of filter paper was placed on bacterial agar plate and covered. the agar plate was placed in a dark area and left alone for seven days. after the seven days was up, alice measured the diameter of the bacterial rings around each piece of filter paper.
the statement that best describes alice’s experiment is
a
the size of the filter paper is serving as a control.
b
the experiment should test only one type of mouthwash.
c
the experiment is well designed and has a valid control.
d
the experiment has too many variables that are being manipulated.

Explanation:

To determine the correct statement about Alice's experiment:

• Option A: The filter paper's size isn't a control; the control is the paper soaked in plain water (to compare with mouthwashes). So A is incorrect.
• Option B: The experiment tests three mouthwashes (independent variable) and has a control (plain water), so testing one mouthwash isn't necessary. B is incorrect.
• Option C: The experiment has one independent variable (mouthwash type), a control (plain water), and measures bacterial rings (dependent variable). Other factors (filter paper size, soaking time, incubation conditions) are constant. So it's well - designed with a valid control. C is correct.
• Option D: The only manipulated variable is the mouthwash type (three types) plus the control. There aren't too many variables. D is incorrect.

Answer:

C. The experiment is well designed and has a valid control.