Context: Read the text.
When immunologists develop a new vaccine, years of testing may be required before it is introduced to humans. Early testing may occur in vitro, or "in glass." In vitro testing is done outside of organisms, in a petri dish or test tube, and can help researchers identify the mechanism by which a vaccine works against a virus.
Another important form of testing that precedes human trials is in vivo testing. In vivo means "within the living." In vivo testing helps demonstrate how a vaccine works within the complex system that is a living organism; for example, a mouse might be given a vaccine and then exposed to a virus to show that the vaccine is effective.
Which term matches the picture?
A. in vivo
B. in vitro
Pick the correct answer from the listed options and provide the letter of the selected option.

B