生物教师利用幻灯或视频资料，可以观察到动植物的缓慢生长过程，这种直观的手段是（）。

A．实物直观

B．模象直观

C．语言直观

D．虚拟直观

By the time he realizes he ______ into a trap, it'll be too late for him to do anything about it. [      ]

A. walks

B. walked    

C. has walked      

D. had walked