How does the group escape the mountain while the dragon is gone?

b.another secret passage ( I think!)

This seems like a trick question to me. The first time the dragon leaves, the dwarves don't escape which would be D however they do hide in a secret passage. But when Smaug goes to Lake Town, the dwarves go out the front gate(C), But it's not really an escape I don't think (D again) . They were too elated about their treasure.