31. What makes the Mariana Trench so mysterious?
(A) The presence of underwater volcanoes
(B) Its vast size and depth
(C) The extensive network of underwater caves
(D) The discovery of mythical sea creatures
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統計: A(0), B(3), C(0), D(1), E(0) #3442649
統計: A(0), B(3), C(0), D(1), E(0) #3442649