Everyone knows that________ is dangerous to play with fire, but________ is difficult is to prevent children from playing with fire.
We finished the run in less than half the time ____.
——-Shall we go out for a walk?——Sorry. This is not the right ____ to invite me. I am too tired to walk.
It is clear that the whole world is passing through a social revolution in which a central _____ must be taken by scientists and technologists.
The author of the report is well _____ with the problems in the hospital because he has been working there for many years.
With the change of the economic foundation the entire immense superstructure is _____ rapidly transformed.