A:have
B:am having
C:have had
D:had
226路汽车的车门是由“压缩空气”控制的,它是将一定数量的封闭气体进行压缩后放入汽缸的,压缩后的气体质量____ 、密度____ (选填:变大、不变、变小)
In order to improve our spoken English, we should make much in our pronunciation.
下图为我国某景区等高线地形图(单位:米),读图回答问题。