问题描述：
-- hurry mum the bus is coming -- wait a minute dear you --- cross the street until the traffic li
-- hurry mum the bus is coming
-- wait a minute dear you --- cross the street until the traffic lights are green
a\ must
b\ need
c\ must not
d\ need not
说理由!
-- hurry mum the bus is coming
-- wait a minute dear you --- cross the street until the traffic lights are green
a\ must
b\ need
c\ must not
d\ need not
说理由!
问题解答：
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