Bob, Kara, and Mark are partners in the BKM Partnership. Bob is a 40% partner and has a June 30 tax year-end. Kara owns a 40% interest in the partnership and has a September 30 tax year-end, and Mark owns the remaining 20% interest and has an October 31 tax year-end. The partnership does not have a natural business year. What is the required tax year-end for the partnership (if no Sec. 444 election is made)?

A.October 31
B.December 31
C.September 30
D.June 30