His marked personality changes were brought about by a series of unfortunate events.

单项选择题Penicillin was discovered by chance in 1928.

单项选择题The Pop Art of the 1960’s used imagery drawn from the everyday world.

单项选择题By the time the war broke out, most of the people had already left.

单项选择题Many photographers prefer to take pictures at twilight when they can take advantage of the special effects of the setting sun.

单项选择题Psychologists have done extensive studies of how well patients comply with doctors’ orders.