Whether to go to college is optional ______that no student is required to do so as he is required by law in most states of America to go to high school.

单项选择题Money serves as a means of accumulating wealth and as a universal______ of exchange in the highly diversified commercial world.

单项选择题People were afraid to leave their house,______ the police had been ordered to stand by in case of emergency, they were just as confused and helpless as anybody else.