Shawna knows that a correlation between calories of sweeteners consumed and mortality rates among women does not, in itself, mean that drinking more soft drinks will lead to a higher chance of her dying.

Which of the following would NOT be a lurking variable in the above scenario?

• a.) The time of day Shawna exercises
• b.) The number of soft drink brands available for purchase
• c.) Shawna's weight
• d.) The amount of caffeine available in soft drinks