15. Two years ago, sisters Donna and Amy, purchased real property and titled it as joint tenants with rights of survivorship. Amy paid the entire purchase price of $100,000. Over the next two years, Donna and Amy allocated the income and expenses of the property equally, and the value of the property increased to $700,000. If Amy dies this year, how much will be included in her gross estate as the value of the real property?

a. $100,000 b. $350,000 c. $600,000 d. $700,000