When he leaves the secret garden for the first time, why does Dickon leave Mary a picture of a missel thrush on its nest?

A. to tell her that he will be at work in the garden before the birds wake up

B. to remind her to move more slowly and softly when she is in the garden

C. to suggest that she build more nests for the birds to live in

D. to let her know that her secret is safe with him

The book is The Secret Garden