D transferred property to T, as trustee, in trust, the income to be paid to X for X's life, Remainder to Y if living at X's death, or if not,...

D transferred property to T, as trustee, in trust, the income to be paid to X for X's life, Remainder to Y if living at X's death, or if not, remainder to Z or Z's estate.  In which of the following situations would an interest in the trust be includible in the gross estate of the person who dies?

A. Y dies survived by X.

B. X dies.

C. Z dies survived by X and Y.

D. T dies.