39.
(A) how many greenhouse gases there are
(B) how long greenhouses have been used
(C) what a greenhouse is and how it works
(D) what greenhouse gases are and where they come from
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統計: A(27), B(43), C(229), D(65), E(0) #2716778
統計: A(27), B(43), C(229), D(65), E(0) #2716778