单选题
Ships traveling in the North Atlantic during the winter must be constantly watchful to avoid icebergs, large masses of ice ______ only one-ninth is visible above water.
【正确答案】
D
【答案解析】large masses of ice是iceberg的同位语，关系代词which的先行词是ice，相当于only one-ninth of the ice is visible above water，所以D是正确答案。so that引导目的状语从句，in that引导原因状语从句，均与句义不符。