Know (something) inside out

Do you know the English expression to know (something) inside out? Read the conversation below. Can you guess the meaning?

Terry: I need help with my science homework.

Gill: Why don’t you ask Sarah? She knows the subject inside out. She has a degree in chemistry.

Does it mean:

a) know something very well

b) know a little about something

c) not know anything

The answer is below! ↓

adult biology chemical chemist

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Answer: a) know something very well