The physics teacher once said that we first _________ the lightning followed by thunders because light _________ faster than sound．
A.see; travelledB.saw; travelledC.saw; travelsD.see; travels在线课程C
解析：
第一个空用saw与主语的时态一致；
第二个表示客观真理，仍然要用一般现在时。