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．that | B．it | C．this | D．what |
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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．that | B．it | C．this | D．what |
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