7. The scientists are pushing the _____ of what’s possible in space exploration, trying to go beyond the normal limits.
(A) letter
(B) package
(C) envelope
(D) container
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統計: A(13), B(15), C(32), D(25), E(0) #3421542
統計: A(13), B(15), C(32), D(25), E(0) #3421542