When it ______ research into heart disease and its effects on the body, we do not have adequate substitutes for the use of animals.
From this story, you can have some idea ______ can be used in reality.
We believe ______ you have been devoted to ______ naturally of great necessity.
______, I can spare no time for it now. I’ll have to finish my homework first.
Can you explain ______ that the medicine has for my illness?
--- I serve three meals every day and do almost everything for him, but he still lets me down.--- I’m afraid that’s ______ you are mistaken.