从A、B、C、D四个选项中，选出可以填入空白处的最佳选项。
21. At first, it seemed that everything went well, but all of ______ sudden, the ship hit ______ iceberg.
A. a; an B. a; the C. the; the D. /; an
22. We have made ______ clear that we are strongly against smoking in the office.
A. this B. that C. it D. one
