单选题 People will find themselves suffering from headache and watering eyes, and even snowhlindness, when______to several hours of "snow light".

【正确答案】 C

【答案解析】相当于when they are exposed to…在when，if unless，though，while，once引导的从句中，如省略，必须将主语与be动词一起省掉。另外，“人们被置于雪光中”，是被动语态，所以D是错的。