方位介词 in, on, under — 英语情景题

情景对话：Sam 和 Lily 在教室里谈论物品的位置。请从 in, on, under 中选择正确的介词填空。 Sam: Look at my desk! My pencil case is (1)________ the desk. Lily: I see it. And your ruler? Sam: The ruler is (2)________ the pencil case. It is inside it. Lily: Oh! Look under your chair. Your ball is (3)________ the chair!