Geologists (at) the Hawaiian Volcano Observatory (rely on) (a number of) instruments to (studying) the volcanoes in Hawaii.
A.at
B.rely on
C.a number of
D.studying

A.A.at
B.rely
B.a

【参考答案】

D
解析：改为study。句中to是不定式短语标志，后应接动词原形。