20(b)
Please refer the link below and find the third answer:
https://www.meritnation.com/ask-answer/question/q3-when-is-a-recessive-trait-capable-of-expressing-it/heredity-and-evolution/3900894
https://www.meritnation.com/ask-answer/question/q3-when-is-a-recessive-trait-capable-of-expressing-it/heredity-and-evolution/3900894