Is the best example of a sensitive period. a. an egg being fertilized b. a child learning to speak a second language c. a child being born blind d. a fetus developing fingers and toes

The best example that illustrates a sensitive period is letter b, where the child tries to learn in speaking a second language because this is considered to be a sensitive period in which is the period for a development of new opportunities that a child could learn in which learning to speak a second language is considered to be a sensitive period.  

the answer to you question is   jet streams

the answer is d.) homologous; have mammary glands