D. I don't know if that is what your teacher will agree with but in my own personal research I have found that to be very effective.
so babies can explore without fear of choking or eating something harmful
I believe that the answer is A: Determined by reinforcement
Answer:
Some vaccines contain whole bacteria or viruses. In these cases the bacteria or viruses will either be severely weakened (attenuated) so that they cannot cause disease in healthy people, or killed altogether (inactivated). Many vaccines contain only parts of viruses or bacteria, usually proteins or sugars from the surface. These stimulate the immune system but cannot cause disease
Explanation:
