—How was your recent trip to Jiuzhaigou?
—I've never had one before.
A:a tired
B:a more tiring
C:a most tiring
D:the most tired
地壳的厚度约为8到40km,在地表以下不太深的地方,温度可按y=3.5x+t计算,其中x是深度,t是地球表面温度,y是所达深度的温度.
世界上黑种人的故乡是