_______ difficult and challenging the task may be, we must finish it on time, because there is a possibility _______our work will go down in history.
When the man came in, gun in hand, we all stood there, ________.
If nature does not provide man with the necessary material, it is the laboratory ____ he will turn to for it.
By the time you have completed the essential training, you _____ exposed to virtually every new feature of the course.
_______ more to be pitied than blamed.
Earth is believed ______ by an object the size of Mars at some point in the distant past before its surface cooled.