Only by practising a few hours every day________ be able to master the language.

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________ is known to us all , God helps those who help themselves.

But I've never even heard of that.

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New policy was carried out; ________, the rich became richer, the poor were even poorer.

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The quarrel ________with the important work left undone.

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________ this book can be used for reading has not been decided yet.

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It was more than fifteen years ago ________ I entered the laboratory of Professor Agassiz.

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----I don' t think I can walk any further.

---- ________ , let's stop here for a rest.

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I'm leaving now.

________ you lock up.

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Do you smoke?

No , but I ________.

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Not only ________ about the food, he also refused to pay for it.

[　　]

A．the customer complained

B．did the customer complain

C．when the customer complained

D．the customer did complain