21. Because industry and human activities are producing enormous amounts of gases, the Earth is experiencing something _____ as the greenhouse effect.
(A) known
(B) knowing
(C) being known
(D) to know

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統計： A(100), B(3), C(23), D(1), E(0) #597934

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B1 · 2015/04/12

#1029120

Because industry and human activities are producing enormous amounts of gases, the Earth is experiencing something _____ as the greenhouse effect.