----Can he work out the maths problem?

----I think _____ hard for him to work it out.

A．it’s

B．that’s

C．this is

D．they’re

地图上通常用地图符号的图形、色彩的变化来表示事物的质量特征；用地图符号的尺寸变化来表示事物的数量特征。