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It never occurred to humans until the mid-1990s          some animals can be cloned.

A. where    B. that             C. when         D. what

It never occurred to humans until the mid-1990s ______ some animals can be cloned.

A. where                B. that                 C. when                 D. what

 It never occurred to humans until the mid-1990s ______ some animals can be cloned.

A. where                      B. that                         C. when                              D. what

It never occurred to humans until the mid-1990s ______ some animals can be cloned.

A. where                B. that                 C. when                 D. what

 It never occurred to humans until the mid­1990s ________ some animals can be cloned.

A．where             B．that                    C．which          D．what

It never occurred to humans until the mid-1990s _____ some animals can be cloned．

A．where         B．that          C．when            D．what

 It never _____  humans until the mid­1990s that some animals can be cloned.