问题
选择题
Now we can fly to Tokyo. There was a time ________ we had to take a boat.
A．how
B．what
C．why
D．when
答案
答案：D
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Now we can fly to Tokyo. There was a time ________ we had to take a boat.
A．how
B．what
C．why
D．when
答案：D
场的变化周期为多少？