______ volcanoes for many years, I am still amazed at their beauty as well as their potential

to cause great damage. [ ]

A. Studied

B. Having studied

C. Studying

D. To have been studied

锅炉的工作过程包括燃料的燃烧过程、烟气向水的传热过程和（）过程。

A、水的受热和汽化

B、烟气的流动

C、烟气的排出

D、汽水混合物的形成