英语单项选择一道() with the size of the whole earth, the biggest oce

英语单项选择一道
() with the size of the whole earth, the biggest ocean does not seem big at all.
A compare
B when comparing
C comparing
D when compared
答案给的是D 为什么选C 现在分词做状语不行
when+xx这个结构怎么用