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.
This investigation is not only one that is continuing and worldwide ______ we expect to continue for quite some time.

A. but one also that

B. but one that also

C. but also one that

D. but that also one