两侧对称的动物有哪些体轴？果蝇胚胎前后轴的建立和分化的机制是什么？
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What does Zhang Ling want to do? ______
A. To book a room.
B. To check in.
C. To pay the bill.
D. To check out.
What should be returned to the hotel when a guest checks out?______
A. The room card.
B. The bill.
C. The luggage.
D. The shuttle bus.
The tour guide has to______ before the guests leave.
A. check in
B. sign the bill
C. check the bill
D. pay for the bill
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