The given sketch depicts a virus in a weakened state. Which of the following sketches represents the safest type of vaccines? 
Definitions:
Longitudinal Study
An observational research method in which data is gathered for the same subjects repeatedly over a period of time.
Cross-sectional Study
A Cross-sectional Study is an observational research design that analyzes data from a population, or a representative subset, at a specific point in time.
Double-blind
An experimental procedure in which neither the participants nor the experimenters know who is receiving a particular treatment, to prevent bias in research results.
Case History
A detailed narrative record or report of a person's background, medical history, or behavioral patterns.
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