A.

is

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are

C.

be

D.

have been

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A.

used

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use

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been used

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have been used

A.

is

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are

C.

were

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is been

A.

pointed

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has pointed

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is pointed

D.

point

A.

are used

B.

is used

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have been used

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been used

A.

are known

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been known

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be known

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have been known

A.

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take

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are taking

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have taken

A.

is found

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been found

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was found

D.

has been found

A.

be established

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established

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is established

D.

was established