压缩机内有异音或产生振动的原因有（）。

A、轴瓦间隙过大；

B、气流通道有杂物；

C、转子径向移动大叶轮和外壳相碰；

D、轴位移过大。

What is the point the speaker is trying to make about technology development
a. A mature technology requires less innovation.
b. Technical evolution is close to maturation in certain fields.
c. New types of energy are expected to mature in the near future.
d. Nanomaterials or photonics and carbon nanotubes are environmentally friendly.