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

When ______ comes to research into heart disease and its effects on the body, we do not have adequate substitutes for the use of animals.

1. A.
   it
2. B.
   this
3. C.
   that
4. D.
   which

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

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

When ______ comes to research into heart disease and its effects on the body, we do not have adequate substitutes for the use of animals.