______ 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

标准麻花钻刃磨时，一般采用粒度为46～80、硬度为中软的（）砂轮为宜。

A、氧化铝

B、碳化物

C、金刚石

D、氧化铝或金刚石