Mark the letter A, B, C or D on...

Câu hỏi: Mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the following questions.
Not only______to determine the depth of the ocean floor, but it is also used to locate oil.

A. seismology is used

B. using seismology

C. is seismology used

D. to use seismology