5. When vessels sail along the coasts, ______ observations at interval are required to
safeguard the safety of navigation.
(A) regular
(B) odd
(C) queer
(D) casual
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統計: A(429), B(23), C(46), D(98), E(0) #3209670
統計: A(429), B(23), C(46), D(98), E(0) #3209670