Question

a scientific study looked at the effect of tanning beds on dna damage. the scientists took skin cells and exposed them to uv radiation (the type used in indoor tanning beds) for different lengths of time: some for 1 minute, some for 5 minutes, some for 15 minutes, and some for 30 minutes. they then looked for signs of dna damage and compared the results to cells that had never been exposed to uv light. what is the independent variable in this experiment?
a the number of skin cells
b the type of light
c the length of time the cells are exposed to uv light
d the untreated cells
e the amount of dna damage in the cells
2.5 points
question 15
a scientist wants to study the effect of vitamin c on colds. he recruits 100 people with colds and gives the experimental group 1000 mg of vitamin c per day. what would be an appropriate control?
a give the control group nothing.
b give the control group 2000 mg of vitamin c per day.
c give the control group orange juice every day.
d give the control group a pill similar to vitamin c but containing sugar (a placebo).
e give the control group 1000 mg of another brand of vitamin c per day.

Explanation:

First Question (Independent Variable in Tanning Bed Experiment)

The independent variable is the factor manipulated by the experimenter. In this study, the scientists changed the length of time skin cells were exposed to UV light (1, 5, 15, 30 minutes). Option A (number of skin cells) is not manipulated. Option B (type of light) is constant (UV from tanning beds). Option D (untreated cells) is a control group, not a variable. Option E (DNA damage) is the dependent variable (measured outcome). So the independent variable is the exposure time.

A control group should differ from the experimental group only in the independent variable (vitamin C here). A placebo (a pill similar in appearance but with sugar) ensures the control group has the same experience (taking a pill) without the active ingredient, avoiding placebo effect. Option A (nothing) might have confounding factors (e.g., psychological effects of no treatment). Option B (2000 mg) is a different dosage, not a control. Option C (orange juice) introduces other variables (sugar, other nutrients). Option E (another brand) still has vitamin C, so the variable isn't isolated. Thus, a placebo pill is appropriate.

Answer:

C. the length of time the cells are exposed to UV light

Second Question (Control for Vitamin C and Colds Experiment)