题文
When ______ comes to research into heart disease and its effects on the body, we do not have adequate substitutes for the use of animals.
| A．it | B．this | C．that | D．which |
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When ______ comes to research into heart disease and its effects on the body, we do not have adequate substitutes for the use of animals.
| A．it | B．this | C．that | D．which |
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