The key is “than”?

涂于德

I was so surprised that he returned home much earlier _____ was expected.
A. as B. than C. which D. /

The key is “than”? How to paraphrase “than” in English?
And what’s the function of “than”?

曼迪

You think "than" is wrong? Maybe right.
"Than"---sth. is compared with another.
It serves as a kind of adverbial component, introduing the object of comparison.

Actually, I'm not sure about the above three questions, just for a discussion.