一幅灰度图像，若每个像素有8位像素深度，则最大灰度数目为（）。
A．128
B．256
C．512
D．1024
参考答案：B
Helen _______her keys in the office so she had to wait until her husband ___home.
A．has left; comes
B．left, had come
C．had left; came
D．had left; would come
知觉恒常性
