Ten subjects are being studied to test the effectiveness of itch relief for three types of medication: A, B, and C. Using a 10-point scale, the researcher calculates the subjects' amount of itching before treatment compared with the amount of itching after treatment. The response variable is the itch rating after one hour of treatment. Previous studies have shown the effects of treatment may vary based on a subject's pain resistance. How should the subjects be blocked? Explain.

They should be blocked alphabetically so the subjects are as different as possible.
They should be blocked by age because older subjects tend to itch more.
They should be blocked by gender because female subjects tend to itch more.
They should be blocked by pain resistance so the subjects are as similar as possible.
Blocking should not be used in this experiment.