题目

—Where do ____ Smiths spend their weekends?—They often go to play on ________ same beach. A . the;the B . a;the C . the;an D . a;an 答案：A电闪雷鸣时，我们总是先看到闪电后听到雷声，说明光速比声速 （大/小）；真空中的光速为 m/s．