问题补充：
from the top of the tower, the south foot of the mountain is a sea of trees.
A. Seen
B. Seeing
C. Having seen
D. To see
答案：
(答案→)A. Seen
B. Seeing
C. Having seen
D. To see
解析：
这题考查过去分词做状语的用法：动词see和主语the south foot of the mountain之间是被动关系。所以用过去分词。句意是：从塔顶上看，山脚的南面是一片树的海洋。选A。
