At 6 months, annabelle dropped her rattle in a fairly rigid way. by 12 months, she tossed objects down the basement stairs, bounced them off walls, and threw them in the air. annabelle's modifications of her dropping scheme are an example of?

The answer to this is letter C, the third statement. 

"In trials, vaccine recipients experienced 85% less outbreaks than non-recipients." is a proof of the efficiency and effectiveness of the vaccine when one takes it. Everybody is trying to avoid the outbreaks and getting sick in the process so having an assurance that the vaccine will work makes people want to take it.