________ did Arthur realize that there was danger.

In mathematics, a variable is a symbol ________ some element of a set.

Few natural elements exist in ________ that they are rarely seen in their natural environments.

The janitor refused to unlock the classroom door because he ________ busy.

________, which is the highest in the range, appears challenging.

The student managed to pass the entrance examination ________ he was blind.

John F.Kennedy was the youngest president of the United States and ________ to be assassinated.

________ the formulation of explanatory laws, the first step in scientific research is the collection and description of facts.

It is earth's gravity that ________ people their weight.

_______ that new information to anyone else but the sergeant.