问题补充：
Their coming without notice really our difficulties.
A. added
B. added up
C. was added to
D. added to
答案：
A. added
B. added up
C. was added to
(答案→)D. added to
解析：add可以作为及物动词，构成“add...to”，表示“把……加到……上”；也可以用作不及物动词，构成短语 add to...，表示“增添，添加”； add up to则表示“共计”。
