Never should we forget about the war and sufferings ----caus

选A.it
it caused to the people there是定语从句,修饰sufferings.
但是这个定语从句的关系词that/which省略了,
因为关系词that/which在定语从句中作caused的宾语(即:it caused sufferings to the people there) ,
因此,可以省略that/which.
为什么不选B或者是C呢?
因为此定从缺少主语及宾语,如填that/which,那么关系词that/which就是作句子的主语,
这样一来句意就讲不通;如把that/which当宾语,那么定从就没有主语,结构就不完整.
所以,不能选B或C.