Parent transfers stock worth $5,000 outright to Child. When the stock has appreciated in value to $10,000, Child sells it and uses the proceeds to purchase other stock worth$10,000 in joint tenancy between Parent and Child. Parent predeceases Child when the jointly owned stock is worth $20,000. What portion of the stock, if any, will be includible in parent's gross estate?

A. $20,000

B. $15,000

C. $10,000

D. $-0-