18. For over a decade, U.S. military and counterterrorism officials have warned that Africa was poised
to become the next frontier _____ international terrorist organizations.
(A) out
(B) for
(C) on
(D) at
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統計: A(4), B(249), C(42), D(34), E(0) #2746599
統計: A(4), B(249), C(42), D(34), E(0) #2746599