--You know the exact date ________ you posted the letter, don’t you?  

--No. But I remember it was a holiday, _______ happened to be Mother’s birthday.  

1. A.
   when; which
2. B.
   that; it
3. C.
   which; that
4. D.
   that; when

--You know the exact date ________ you posted the letter, don’t you?

-- No. But I remember it was a holiday, _______ happened to be Mother’s birthday.

[　　]

--You know the exact date ________ you posted the letter, don’t you?

-- No. But I remember it was a holiday, _______ happened to be Mother’s birthday.

[　　]

－You know the exact date ________ you posted the letter, don't you?

－No. But I remember it was a holiday ________ happened to be Mother's Day.

[　　]