Not until the early years of the 19th century ________ what heat was.
The poster says that a writer and poet ___ to give us a talk on English next week.
How this kind of plant can be ____ in a special way is still unknown to us.
If I ___ free tonight, I'll go to the cinema with you.
Edison had built a chemical lab of ___ when he was very young.
This is the only one of the books on space ___.