For "qualifying widow(er)" filing status, which of the following requirements must be met?
I. The surviving spouse does not remarry before the end of the current year
II. The surviving spouse was eligible to file a joint tax return in the year of the spouse's death
III. The surviving spouse maintains the cost of the principal residence for six months.
a. I, II, and III
b. I and II, but not III
c. I and III, but not II
d. I only