Yes you can use we to refer to the audience, with you in it, because using we groups together a group, like "We the people..."
Your answer is: Yes.
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Since this drawing is his way of reminding her that she and their garden are "as safe as a missel thrush", it makes her one of the many creatures whose secrets he keeps faithfully. So, having that in mind, I would say the correct answer is A) his trustworthiness.
