_______caused the accident is still a complete mystery.
She knows that as a secretary she must be pleasant and helpful no matter how busy she is or what kind of ____ she may be in.
— ____ sweep the classroom yesterday?—I was playing tennis with my friend.
He didn’t _____ to the news.
In the Great Depression, the American government couldn’t find good solutions _____ the unemployment problem.
In January, 2004, the United States successfully launched “Spirit”, a Mars Exploration Rover, _____ a new milestone in the history of mankind.