单项选择题
A．for B．in order to C．as D．with
A．they wailed their houses with glass B．they leave the office doors open while working C．they never have fence D．they depended upon each other
A．Psychologists do not yet know how people see. B．The best experiments are those in which all factors are controlled. C．The study of perception is going on now. D．Perception does not involve psychological factors.