自动照相机能够根据光的强弱自动调整曝光量，所用的感光元件就是一个用半导体材料制成的光敏电阻。影响光敏电阻导电性能的主要因素是 [     ]

A．温度           

B．光照              

C．电压            

D．压力

I was doing some shopping in the supermarket ________ it suddenly occurred to me that I'd left

my wallet at home.[ ]

A. when

B. while

C. until

D. before