5. 11:00 由於風浪大，主機轉速從90轉減為80轉，其英文翻譯為11:00 Main engine speedreduced from 90 rpm to 80 rpm owing to a ___________.，其空格為下列何者？
(A) calm sea.
(B) moderate sea.
(C) mild sea
(D) rough sea.